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/metadata/di_spec.ts

@@ -9,7 +9,7 @@
 import {Component, Directive, ElementRef, Input, NO_ERRORS_SCHEMA, QueryList, ViewChild, ViewChildren} from '@angular/core';
 import {TestBed} from '@angular/core/testing';
 
-export function main() {
+{
   describe('ViewChild', () => {
     beforeEach(() => {
       TestBed.configureTestingModule({