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/src/output/source_map_util.ts

@@ -0,0 +1,39 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {SourceMap} from '@angular/compiler';
+const b64 = require('base64-js');
+const SourceMapConsumer = require('source-map').SourceMapConsumer;
+
+export interface SourceLocation {
+  line: number;
+  column: number;
+  source: string;
+}
+
+export function originalPositionFor(
+    sourceMap: SourceMap,
+    genPosition: {line: number | null, column: number | null}): SourceLocation {
+  const smc = new SourceMapConsumer(sourceMap);
+  // Note: We don't return the original object as it also contains a `name` property
+  // which is always null and we don't want to include that in our assertions...
+  const {line, column, source} = smc.originalPositionFor(genPosition);
+  return {line, column, source};
+}
+
+export function extractSourceMap(source: string): SourceMap|null {
+  let idx = source.lastIndexOf('\n//#');
+  if (idx == -1) return null;
+  const smComment = source.slice(idx).trim();
+  const smB64 = smComment.split('sourceMappingURL=data:application/json;base64,')[1];
+  return smB64 ? JSON.parse(decodeB64String(smB64)) : null;
+}
+
+function decodeB64String(s: string): string {
+  return b64.toByteArray(s).reduce((s: string, c: number) => s + String.fromCharCode(c), '');
+}