[Infrastructure] - Make iOS tests use selection heuristics

[angelosilvestre]

On iOS, we have some selection heuristics to place the caret at word boundaries when tapping at the middle of a word. However, on tests this is disabled by default with the following comment:

// By default we don't use iOS-style selection heuristics in tests because in tests
// we want to know exactly where we're placing the caret.

While this makes sense for some tests, tests that care about interacting with the caret should use these heuristics. One example is #2152, where we have a test to make sure the toolbar is displayed when tapping on the caret, but the test didn't catch the bug because it wasn't using the selection heuristics.

We should modify tests that tap on the caret to use the iOS selection heuristics by default.

[angelosilvestre]

cc @matthew-carroll