Wait for module UI test buttons to be hittable before tapping them

[jmagman]

Another attempt at deflaking the module_test_ios tests. Wait for the buttons to both exist and be "hittable" before tapping. I wasn't able to reproduce these flakes locally.

See also #85650

Speculatively fixes #87478

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• All existing and new tests are passing.

[gaaclarke]

Sounds plausible, factor out the helper function please.

[gaaclarke]

helper function

-(void)waitForElement:(XCUIElement*)element {
  NSPredicate *hittable = [NSPredicate predicateWithFormat:@"exists == YES AND hittable == YES"];
  [self expectationForPredicate:hittable evaluatedWithObject:element handler:nil];
  [self waitForExpectationsWithTimeout:30.0 handler:nil];
}

[gaaclarke]

LGTM!