getByTestId method
Finds the first ElNode in the tree whose data-testid attribute matches testId.
Throws StateError if no matching element is found.
Implementation
ElNode getByTestId(String testId) {
final found = queryByTestId(testId);
if (found == null) {
throw StateError('BloomTest: No element found with data-testid="$testId".');
}
return found;
}