Add test.Helper, also works with Go 1.8

internal/test/helper.go

@@ -0,0 +1,15 @@
+// +build go1.9
+
+package test
+
+import "testing"
+
+// Helperer marks the current function as a test helper.
+type Helperer interface {
+	Helper()
+}
+
+// Helper returns a function that marks the current function as a helper function.
+func Helper(t testing.TB) Helperer {
+	return t
+}

internal/test/helper_go18.go

@@ -0,0 +1,19 @@
+// +build !go1.9
+
+package test
+
+import "testing"
+
+// Helperer marks the current function as a test helper.
+type Helperer interface {
+	Helper()
+}
+
+type fakeHelper struct{}
+
+func (fakeHelper) Helper() {}
+
+// Helper returns a function that marks the current function as a helper function.
+func Helper(t testing.TB) Helperer {
+	return fakeHelper{}
+}