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/testing/BUILD.bazel

@@ -8,5 +8,8 @@ ts_library(
     srcs = glob(["**/*.ts"]),
     module_name = "@angular/compiler/testing",
     tsconfig = "//packages:tsconfig",
-    deps = ["//packages/compiler"],
+    deps = [
+        "//packages:types",
+        "//packages/compiler",
+    ],
 )