cmd/testwrapper: surface race reports and skip retries when detected

A data race in a package matters more than any individual test
result. Two related problems:

1. Where go test's race detector text ("WARNING: DATA RACE" plus
   the goroutine stack traces) lands in JSON output is timing-
   dependent: it can be attributed to a test that ends up reporting
   PASS (e.g. when the racing goroutines outlive the test that
   spawned them and TSan prints during a different test's window).
   testwrapper's main loop only flushes the logs of failed tests,
   so the race report ends up stuck in a passing test's buffer and
   is silently dropped. The race builders just see a bare
   "FAIL\nFAIL\tpkg\ttime".

2. If the failing test in such a package happens to be marked flaky,
   testwrapper retries it. That is the worst possible response to a
   race: the flaky test might not even be the racy code, and a
   second run without the racy goroutines could "succeed" while
   hiding the real bug.

Address both: scan every output line for the race detector's first-
line marker. Track whether the package observed a race at all, on
the pkgFinished testAttempt. When a race was seen, fold every per-
test log buffer into the package-level logs (so the full report
surfaces from the existing pkg-fail flush path), and drop any
flaky-test retry plans for that package so we fail immediately
instead of running another attempt.

Two new tests:
- TestRaceSuppressesFlakyRetry verifies that a flaky test alongside
  a racy test does NOT get retried.
- TestRaceAttributedToPassingTest verifies that a race attributed by
  test2json to a passing test still surfaces in the output.

Also add a corpus of captured raw test binary outputs under
cmd/testwrapper/testdata/, with one subdirectory per scenario,
documenting the six representative shapes that go test -race can
emit (race in test body, race in goroutines that outlive a test,
race forced into a later test, race in TestMain post-m.Run, and a
parallel-tests split-attribution case via a "=== NAME" redirect
line). See its README.md for details.

Fixes #19603

Change-Id: Ifbfcd67fb3b1882c4907bd9cb2d68a8b5a91dd54
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/testwrapper/testdata/C_spawnwait/caught.raw.txt

@@ -0,0 +1,32 @@
+=== RUN   TestSpawn
+==================
+WARNING: DATA RACE
+Read at 0x0000007f12a8 by goroutine 9:
+  racesurvey/C_spawnwait.TestSpawn.func2()
+      /tmp/racesurvey/C_spawnwait/c_test.go:14 +0x70
+
+Previous write at 0x0000007f12a8 by goroutine 8:
+  racesurvey/C_spawnwait.TestSpawn.func1()
+      /tmp/racesurvey/C_spawnwait/c_test.go:13 +0x88
+
+Goroutine 9 (running) created at:
+  racesurvey/C_spawnwait.TestSpawn()
+      /tmp/racesurvey/C_spawnwait/c_test.go:14 +0x44
+  testing.tRunner()
+      /home/ubuntu/sdk/go1.26.3/src/testing/testing.go:2036 +0x21c
+  testing.(*T).Run.gowrap1()
+      /home/ubuntu/sdk/go1.26.3/src/testing/testing.go:2101 +0x38
+
+Goroutine 8 (finished) created at:
+  racesurvey/C_spawnwait.TestSpawn()
+      /tmp/racesurvey/C_spawnwait/c_test.go:13 +0x34
+  testing.tRunner()
+      /home/ubuntu/sdk/go1.26.3/src/testing/testing.go:2036 +0x21c
+  testing.(*T).Run.gowrap1()
+      /home/ubuntu/sdk/go1.26.3/src/testing/testing.go:2101 +0x38
+==================
+    testing.go:1712: race detected during execution of test
+--- FAIL: TestSpawn (0.00s)
+=== RUN   TestWait
+--- PASS: TestWait (0.00s)
+FAIL

cmd/testwrapper/testdata/C_spawnwait/pass.raw.txt

@@ -0,0 +1,31 @@
+=== RUN   TestSpawn
+--- PASS: TestSpawn (0.00s)
+=== RUN   TestWait
+==================
+WARNING: DATA RACE
+Read at 0x0000007f12a8 by goroutine 8:
+  racesurvey/C_spawnwait.TestSpawn.func1()
+      /tmp/racesurvey/C_spawnwait/c_test.go:13 +0x70
+
+Previous write at 0x0000007f12a8 by goroutine 9:
+  racesurvey/C_spawnwait.TestSpawn.func2()
+      /tmp/racesurvey/C_spawnwait/c_test.go:14 +0x88
+
+Goroutine 8 (running) created at:
+  racesurvey/C_spawnwait.TestSpawn()
+      /tmp/racesurvey/C_spawnwait/c_test.go:13 +0x34
+  testing.tRunner()
+      /home/ubuntu/sdk/go1.26.3/src/testing/testing.go:2036 +0x21c
+  testing.(*T).Run.gowrap1()
+      /home/ubuntu/sdk/go1.26.3/src/testing/testing.go:2101 +0x38
+
+Goroutine 9 (finished) created at:
+  racesurvey/C_spawnwait.TestSpawn()
+      /tmp/racesurvey/C_spawnwait/c_test.go:14 +0x44
+  testing.tRunner()
+      /home/ubuntu/sdk/go1.26.3/src/testing/testing.go:2036 +0x21c
+  testing.(*T).Run.gowrap1()
+      /home/ubuntu/sdk/go1.26.3/src/testing/testing.go:2101 +0x38
+==================
+--- PASS: TestWait (0.00s)
+FAIL

cmd/testwrapper/testdata/C_spawnwait/src.go

@@ -0,0 +1,22 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package spawnwait
+
+import (
+	"sync"
+	"testing"
+)
+
+var counter int
+var wg sync.WaitGroup
+
+func TestSpawn(t *testing.T) {
+	wg.Add(2)
+	go func() { defer wg.Done(); counter++ }()
+	go func() { defer wg.Done(); counter++ }()
+}
+
+func TestWait(t *testing.T) {
+	wg.Wait()
+}