if err := b.BackgroundShell().Mkdir(testDir); err != nil {
		return nil, nil, nil, err
	}

	pmain.Dir = testDir
	pmain.Internal.OmitDebug = !testC && !testNeedBinary()
	if pmain.ImportPath == "runtime.test" {
		// The runtime package needs a symbolized binary for its tests.
		// See runtime/unsafepoint_test.go.
		pmain.Internal.OmitDebug = false
	}

	if !cfg.BuildN {
		// writeTestmain writes _testmain.go,