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

util/testenv/testenv.go

@@ -22,6 +22,19 @@ func InTest() bool {
 	})
 }
 
+// Verbose reports whether the test binary is running with verbose
+// output (the -test.v flag), like testing.Verbose but without
+// importing the testing package.
+func Verbose() bool {
+	f := flag.Lookup("test.v")
+	if f == nil {
+		return false
+	}
+	// The flag's String value is "false" when off, and "true" or
+	// "test2json" when on.
+	return f.Value.String() != "false"
+}
+
 // TB is testing.TB, to avoid importing "testing" in non-test code.
 type TB interface {
 	ArtifactDir() string