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/src/compiler.ts

@@ -76,4 +76,5 @@ export * from './style_compiler';
 export * from './template_parser/template_parser';
 export {ViewCompiler} from './view_compiler/view_compiler';
 export {getParseErrors, isSyntaxError, syntaxError, Version} from './util';
+export {SourceMap} from './output/source_map';
 // This file only reexports content of the `src` folder. Keep it that way.