Allow being able to run a specific PW test or class for multiple browsers

[Varun Kasyap Pentamaraju]

you can run the full Playwright suite or a module against several browsers at once. But, if you ask for one test class or one test, only the first browser actually runs. The other browsers are ignored.

For example:

python tests/runtests.py --playwright=chromium,firefox,webkit --parallel=1 admin_changelist.tests.PlaywrightTests
Found 10 test(s).

But 10 * 3 = 30 tests should run.

One more example:

python tests/runtests.py --playwright=chromium,firefox,webkit --parallel=1 admin_changelist.tests.PlaywrightTests.test_add_row_selection
Found 1 test(s).

But three tests should run.

This issue is not seen when running the full module:

python tests/runtests.py --playwright=chromium,firefox,webkit --parallel=1 admin_changelist.tests
Found 30 test(s).

After the fix, the same command you already use to target a class or a single test should honor every browser you pass.