test(@angular-devkit/build-angular): add browser builder tsConfig option tests

This change adds expanded unit tests for the browser builder's `tsConfig` option using the builder test harness.

packages/angular_devkit/build_angular/src/browser/tests/options/tsconfig_spec.ts

@@ -0,0 +1,55 @@
+/**
+ * @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 { buildWebpackBrowser } from '../../index';
+import { BASE_OPTIONS, BROWSER_BUILDER_INFO, describeBuilder } from '../setup';
+
+describeBuilder(buildWebpackBrowser, BROWSER_BUILDER_INFO, (harness) => {
+  describe('Option: "tsConfig"', () => {
+    it('uses a provided TypeScript configuration file', async () => {
+      // Setup a TS file that uses ES2015+ const and then target ES5.
+      // The const usage should be downleveled in the output if the TS config is used.
+      await harness.writeFile('src/main.ts', 'const a = 5; console.log(a);');
+      await harness.writeFile(
+        'src/tsconfig.option.json',
+        JSON.stringify({
+          compilerOptions: {
+            target: 'es5',
+          },
+          files: ['main.ts'],
+        }),
+      );
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        tsConfig: 'src/tsconfig.option.json',
+      });
+
+      const { result } = await harness.executeOnce();
+
+      expect(result?.success).toBe(true);
+
+      harness.expectFile('dist/main.js').content.not.toContain('const');
+    });
+
+    it('throws an exception when TypeScript Configuration file does not exist', async () => {
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        tsConfig: 'src/missing.json',
+      });
+
+      const { result, error } = await harness.executeOnce({ outputLogsOnException: false });
+
+      expect(result).toBeUndefined();
+      expect(error).toEqual(
+        jasmine.objectContaining({
+          message: jasmine.stringMatching('no such file or directory'),
+        }),
+      );
+    });
+  });
+});