build(core): add bazel test targets for core (#21053)

- Add tests target for `test`, `test_node_only` and `test_web` in `core` package.
- Created a `_testing_init` pseudo package where bootstrap code for tests is kept.
- Moved `source_map_util` from `test` to `testing` so to prevent circular dependency.
- Removed `visibility:public` for testing `BUILD` packages.

PR Close #21053

packages/compiler/test/output/abstract_emitter_node_only_spec.ts

@@ -9,7 +9,7 @@
 import {ParseLocation, ParseSourceFile, ParseSourceSpan} from '@angular/compiler';
 import {EmitterVisitorContext} from '@angular/compiler/src/output/abstract_emitter';
 import {SourceMap} from '@angular/compiler/src/output/source_map';
-import {extractSourceMap, originalPositionFor} from './source_map_util';
+import {extractSourceMap, originalPositionFor} from '@angular/compiler/testing/src/output/source_map_util';
 
 export function main() {
   describe('AbstractEmitter', () => {