run web tests in batches; enable foundation tests

[yjbanov]

Description

Change the way we run Web tests. Instead of running all tests in a single browser instance, tests are divided into batches (20 tests per batch) and each batch gets its own dedicated browser instance. The browser is shut down and restarted between batches so it does not accumulate state that could potentially get corrupted, leading to flakiness.

This change also runs all foundation tests as a test batch. If it's stable enough we'll enable more tests in subsequent PRs.

Related Issues

#37258

Tests

There no new code to add tests for.

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[jonahwilliams]

LGTM