Fix slow property test

[Unisay]

#6062 (comment)

        parser ignores whitespace:                                                    OK (687.93s)
            ✓ propIgnores whitespace passed 50000 tests.
        parser ignores comments:                                                      OK (2314.43s)
            ✓ propIgnores comments passed 50000 tests.

The whitespace parser tests are also fragile: its easy to break them when changing name representations, e.g. ` symbols etc.

[effectfully]

I've fixed those in #6286.