cmd/testwrapper: auto-retry every failing test

Previously, testwrapper only retried tests explicitly annotated with
flakytest.Mark. Authors don't pre-emptively mark tests that haven't
flaked yet, so the first flake of a brand-new test failed CI even
when a re-run would have passed.

testwrapper now retries every failing test within a per-test wall-clock
budget (default: 5 minute per-attempt timeout capped at 1.5x the first
failure duration, 10 minute total). A test that fails and then passes
on retry is reported as flaky; a test that never passes within the
budget remains a real failure (exit non-zero).

For flakeapp's existing log scraping, the wire format is preserved:
the "flakytest failures JSON:" line is now emitted only for tests
that ultimately flaked (passed on retry). Unmarked tests get a fake
issue URL of the form https://github.com/{owner}/{repo}/issues/UNKNOWN
where owner/repo is detected from GITHUB_REPOSITORY, the local git
remote, or falls back to tailscale/tailscale. A new "permanent test
failures JSON:" line is emitted for tests that never passed; flakeapp
ignores it for now (a follow-up can teach it to record real failures
separately).

flakytest.Mark stays as an opt-in API: still useful for tracking a
known-flaky test against a real issue and for TS_SKIP_FLAKY_TESTS.

Updates tailscale/corp#38960

Change-Id: I56dfc9b023486d239f60793a53e9690578ce8017
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/testwrapper/flakytest/flakytest.go

@@ -1,9 +1,14 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-// Package flakytest contains test helpers for marking a test as flaky. For
-// tests run using cmd/testwrapper, a failed flaky test will cause tests to be
-// re-run a few time until they succeed or exceed our iteration limit.
+// Package flakytest contains test helpers for marking a test as flaky.
+//
+// Marking a test with [Mark] is not required for cmd/testwrapper to retry
+// failed tests; the wrapper retries any failure within a per-test time
+// budget and reports a test as flaky if it ever passes on retry. Mark is
+// useful for tracking a known-flaky test against a GitHub issue and for the
+// TS_SKIP_FLAKY_TESTS skip behavior used to keep CI green when a flake is
+// being investigated.
 package flakytest
 
 import (