tstest, util/testenv: drop tstest's dependency on the testing package

Change tstest's exported functions (AssertNotParallel, Replace,
Parallel, RequireRoot, SkipOnKernelVersions, MinAllocsPerRun, FixLogs,
UnfixLogs, CheckIsZero, ResourceCheck) to take testenv.TB instead of
testing.TB or *testing.T, so importing tstest from non-test code no
longer links the testing package and its flag registration side
effects into the binary. Add testenv.Verbose to replace the one use of
testing.Verbose, and a deptest check to keep testing out of tstest's
dependency graph.

Callers are unaffected: *testing.T and testing.TB both satisfy
testenv.TB.

Updates tailscale/corp#45223

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ib373ff66ceff638d071582baf8367245987e9155

tstest/tstest_test.go

@@ -6,8 +6,18 @@ package tstest
 import (
 	"runtime"
 	"testing"
+
+	"tailscale.com/tstest/deptest"
 )
 
+func TestDeps(t *testing.T) {
+	deptest.DepChecker{
+		BadDeps: map[string]string{
+			"testing": "do not link the testing package and its flag side effects into non-test binaries; use testenv.TB",
+		},
+	}.Check(t)
+}
+
 func TestReplace(t *testing.T) {
 	before := "before"
 	done := false