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/service-worker/worker/test/BUILD.bazel

@@ -1,5 +1,3 @@
-package(default_visibility = ["//visibility:public"])
-
 load("@build_bazel_rules_typescript//:defs.bzl", "ts_library")
 load("@build_bazel_rules_nodejs//:defs.bzl", "jasmine_node_test")