build: bundle spec files for saucelabs legacy job (#44281)

Bundle spec files similar to how it is done within the Angular
Components repo. This should simplify the setup and also speed
up the Saucelab job as only a single spec bundle would need to be
downloaded, compared to having to load hundreds of files through the
Saucelabs tunnel.

Also makes a couple of tests more robust with the emulators/and accounts
for ES2015 test runner changes. The tests should be less reluctant to
such build process changes.

Note for reviewers: Some imports have been simplified here. This work
came from Joey's original WIP for this. It's unclear to me whether this
is still needed, but it sounded like this was necessary for the ESBuild
bundling to work. I have robusted the module resolution plugin though,
so I doubt it's still needed. At the same time though: Not worth
reverting/trying as these changes are nice to have anyway!

Co-Authored-By: Joey Perrott <josephperrott@gmail.com>
Co-Authored-By: Paul Gschwendtner <paulgschwendtner@gmail.com>

PR Close #44281

.circleci/config.yml

@@ -614,9 +614,9 @@ jobs:
                 //packages/common/locales
 
             # Needed for the ES5 downlevel reflector test in `packages/core/test/reflection`.
-            mkdir -p dist/all/@angular/core/test/reflection/
+            mkdir -p dist/legacy-test-out/core/test/reflection/
             cp dist/bin/packages/core/test/reflection/es5_downleveled_inheritance_fixture.js \
-              dist/all/@angular/core/test/reflection/es5_downleveled_inheritance_fixture.js
+              dist/legacy-test-out/core/test/reflection/es5_downleveled_inheritance_fixture.js
             # Locale files are needed for i18n tests running within Saucelabs. These are added
             # directly as sources so that the TypeScript compilation of `/packages/tsconfig.json`
             # can succeed. Note that the base locale and currencies files are checked-in, so
@@ -624,11 +624,10 @@ jobs:
             mkdir -p packages/common/locales/extra
             cp dist/bin/packages/common/locales/*.ts packages/common/locales
             cp dist/bin/packages/common/locales/extra/*.ts packages/common/locales/extra
-      # add module umd tsc compile option so the test can work
-      # properly in the legacy browsers
-      - run: yarn tsc -p packages/tsconfig-legacy-saucelabs.json --module UMD
-      - run: yarn tsc -p modules --module UMD
+      # Build ZoneJS so that it can be loaded globally in the Karma tests.
       - run: yarn bazel build //packages/zone.js:npm_package
+      # Build the bundle for all tests to run within Saucelabs
+      - run: node tools/legacy-saucelabs/build-saucelabs-test-bundle.mjs
       - run:
           # Waiting on ready ensures that we don't run tests too early without Saucelabs not being ready.
           name: Waiting for Saucelabs tunnel to connect
@@ -636,8 +635,7 @@ jobs:
       - run:
           name: Running tests on Saucelabs.
           command: |
-            browsers=$(node -e 'console.log(require("./browser-providers.conf").sauceAliases.CI_REQUIRED.join(","))')
-            yarn karma start ./karma-js.conf.js --single-run --browsers=${browsers}
+            KARMA_WEB_TEST_MODE=SL_REQUIRED yarn karma start ./karma-js.conf.js --single-run
       - run:
           name: Stop Saucelabs tunnel service
           command: ./tools/saucelabs/sauce-service.sh stop