Add karma_web_test and karma_web_test_suite

[gregmagolan]

Adds karma_web_test and karma_web_test_suite which accepts a user base karma configuration in config_file attribute. ts_web_test (a generic web_test rule) uses karma_web_test code under the hood but does not accept a karma configuration. Updated examples/testing:testing to also show karm_web_test usage.

Also some general cleanup in the karma rule.

Prerequisite for angular/angular#27721.

[alexeagle]

could we have the user supply a karma plugin rather than a karma conf? Then we don't have the problem that they provide karma settings that conflict with ours under bazel, requiring warnings like we do for tsconfig overrides or similar

rollup_bundle already allows plugins

[gregmagolan]

Added configuration_env_vars as well so I could make the following work in angular/angular:

$ yarn bazel test //:test_web_all --define=TS_WEB_TEST_MODE=SL_REQUIRED

karma-js.conf.js:

  switch (process.env.TS_WEB_TEST_MODE) {
    case 'SL_REQUIRED':
      conf.browsers = browserProvidersConf.sauceAliases.CI_REQUIRED;
      break;
    case 'SL_OPTIONAL':
      conf.browsers = browserProvidersConf.sauceAliases.CI_OPTIONAL;
      break;
    case 'BS_REQUIRED':
      conf.browsers = browserProvidersConf.browserstackAliases.CI_REQUIRED;
      break;
    case 'BS_OPTIONAL':
      conf.browsers = browserProvidersConf.browserstackAliases.CI_OPTIONAL;
      break;
    default:
      conf.browsers = [process.env['DISPLAY'] ? 'Chrome' : 'ChromeHeadless'];
  }

Test target of SauceLabs, BrowserStack or local chrome controlled by -define=TS_WEB_TEST_MODE=

[gregmagolan]

That's a good question. The plugin would have to be able to be able to override configuration values such as browsers, customLaunchers,sauceLabs, browserStack etc... (see https://github.com/angular/angular/pull/27721/files#diff-c4667ceccd7e91092048e752850fab9c) which would make it functionally the same as a base configuration except harder to setup for users.

[alexeagle]

I assume you already tried to reduce the duplication?

The approach looks good to me, I'll let @kyliau review the karma-specific bits

[alexeagle]

not sure we should put this in the public API yet.

I think generally we should have some process for maturing new rules before they go in the public API

[kyliau]

LGTM