Make frontend unit test code could know it is in testing (#32656)

See the comment of isInFrontendUnitTest

web_src/js/utils/testhelper.ts

@@ -0,0 +1,6 @@
+// there could be different "testing" concepts, for example: backend's "setting.IsInTesting"
+// even if backend is in testing mode, frontend could be complied in production mode
+// so this function only checks if the frontend is in unit testing mode (usually from *.test.ts files)
+export function isInFrontendUnitTest() {
+  return process.env.TEST === 'true';
+}