Extending grammar integration tests (#6644)
* Cleaning up integration tests to share code between tests and make it simpler to add new tests.
* Add tests around quantifiers to ensure both matching and non-matching compliance.
* Add slightly more complex grammar with quantifiers to test references with quantifiers.
* Fixing build when C++17 is not present.
* Separating test calls to give more helpful stack traces on failure. Adding verbose messages to give visibility for what is being tested.
* Adding quotes around strings to explicitly show whitespace
* Removing trailing whitespace.
* Implementing suggestions from @ochafik -- grammars and test strings now print and flush before tests to aid in debugging segfaults and whatnot.
* Cleaning up forgotten symbols. Modifying simple test to use test harness. Added comments for more verbose descriptions of what each test is accomplishing.
* Unicode symbol modifications to hopefully make log easier to parse visually.
