Names of tests in .xcresult don't allow for hierarchy recreation

[bartekpacia]

test file structure:

void main() {
  patrol('at the beginning', ($) async {});

  group('top level group in file', () {
    group('alpha', () {
      patrol('first', ($) async {});
      patrol('second', ($) async {});
    });

    patrol('in the middle', ($) async {});

    group('bravo', () {
      patrol('first', ($) async {});
      patrol('second', ($) async {});
    });
  });
}

xcresult:

We need to delimit group&test names with e.g. _ (and forbid users from using _ in their test names because that'd mess up decoding).