RFC: Add --files to allow running specific test files.

[mkhatib]

Following are test runs for test-amp-iframe.js with the following command:

Test	Single Run	Subsequent Watched Runs
gulp test (describe.only - current flow) gulp test	59s	~8s
gulp test (--files - build all extensions) gulp test --files=extensions/amp-iframe/0.1/test/*.js	36s	~6s
gulp test (--files build only extensions being tested - this change) gulp test --files=extensions/amp-iframe/0.1/test/*.js	18s	~4s

Even with no speed wins this also allows us to run tests that are in multiple files or multiple describe blocks in the same file - currently not possible, one would have to keep doing .only on multiple describe blocks. Speed wins also gonna keep accumulate as we add more and more extensions.

In general this hopefully could be very useful during development where one is just incrementally adding tests and testing functionality in general.

I general this can even be faster with more optimization to what needs to be built or compiled. For example, as far as I can tell one wouldn't need to run compile in build step for running the unit tests, though not entirely sure (for example, the test-amp-iframe runs in 12s instead of 18s if compile is dropped). One can also probably skip building examples during running these kind of tests (though the saves from examples might be insignificant - ~1s - another ~1s for dropping buildAlp when not needed).

I think more importantly, this get rid of a lot of .only usage and forgetting it in source code to only realize the test failed after travis has ran presubmit - some usage still probably useful to run a single describe or it blocks.

cc/ @cramforce @dvoytenko @jridgewell @sriramkrish85 @erwinmombay

[erwinmombay]

just do:

files = [].concat(config.commonTestPaths, argv.files)

[mkhatib]

woah thanks 👍 Done

[erwinmombay]

LGTM