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

tools/cjs-jasmine/index.ts

@@ -52,6 +52,7 @@ var specFiles: any =
                      cwd: distAll,
                      ignore: [
                        // the following code and tests are not compatible with CJS/node environment
+                       '@angular/_testing_init/**',
                        '@angular/examples/**',
                        '@angular/platform-browser/**',
                        '@angular/platform-browser-dynamic/**',