build(core): remove main() from specs (#21053)

`main()` function used to be needed to support dart, since dart
Does not allow top level statements. Since we no longer use dart
The need for `main()` has been removed.

In preparation for `Basel` and standardized way of running tests
we are removing `main()`

PR Close #21053

packages/core/test/testing_internal_spec.ts

@@ -19,7 +19,7 @@ class SpyTestObj extends SpyObject {
   constructor() { super(TestObj); }
 }
 
-export function main() {
+{
   describe('testing', () => {
     describe('equality', () => {
       it('should structurally compare objects', () => {