ktesting: add Parallel

It's useful for writing parallel unit tests.

test/utils/ktesting/tcontext.go

@@ -75,6 +75,15 @@ type TContext interface {
 	context.Context
 	TB
 
+	// Parallel signals that this test is to be run in parallel with (and
+	// only with) other parallel tests. In other words, it needs to be
+	// called in each test which is meant to run in parallel.
+	//
+	// Only supported in Go unit tests. When such a test is run multiple
+	// times due to use of -test.count or -test.cpu, multiple instances of
+	// a single test never run in parallel with each other.
+	Parallel()
+
 	// Cancel can be invoked to cancel the context before the test is completed.
 	// Tests which use the context to control goroutines and then wait for
 	// termination of those goroutines must call Cancel to avoid a deadlock.
@@ -381,6 +390,12 @@ type testingTB struct {
 	TB
 }
 
+func (tCtx tContext) Parallel() {
+	if tb, ok := tCtx.TB().(interface{ Parallel() }); ok {
+		tb.Parallel()
+	}
+}
+
 func (tCtx tContext) Cancel(cause string) {
 	if tCtx.cancel != nil {
 		tCtx.cancel(cause)

test/utils/ktesting/tcontext_test.go

@@ -86,6 +86,23 @@ func TestCancelCtx(t *testing.T) {
 	tCtx.Cancel("test is complete")
 }
 
+func TestParallel(t *testing.T) {
+	var wg sync.WaitGroup
+	wg.Add(3)
+
+	tCtx := ktesting.Init(t)
+
+	// Each sub-test runs in parallel to the others and waits for the other two.
+	test := func(tCtx ktesting.TContext) {
+		tCtx.Parallel()
+		wg.Done()
+		wg.Wait()
+	}
+	tCtx.Run("one", test)
+	tCtx.Run("two", test)
+	tCtx.Run("three", test)
+}
+
 func TestWithTB(t *testing.T) {
 	tCtx := ktesting.Init(t)