[747] Prospective UI test stability improvements

[Clstroud]

This PR doesn't quite normalize the entire suite (yet), but it attempts to improve the underpinnings of the testPagesAdjustToNewPhrases test, which currently fails intermittently. Any other tests that share those underpinnings have been updated to match.

1. throws has been introduced into many helper functions. This may not be strictly necessary, but it allows the Swift code to read more clearly, showing how/when a failure might occur. We can dial that back as it makes sense.
2. tap(afterWaitingForExistenceWithTimeout:) has been replaced with tapWhenExists(timeout: TimeInterval = 0.5). This makes the call site more concise and provides a default timeout value.
3. File/line parameters for XCT* assertions are more consistently propagated to improve diagnostics when a failure does occur. There may be a few calls I missed along the way, but the situation should be generally improved.