moving test-models subdirectories to good and bad

#### Summary:

Note: this should just need a quick spot check. This was just a move of the files and changing bits of files that have to point to the new locations.

The `test-models` for unit testing have been moved to two subdirectories. The good models contain models that are run through the parser and code generator. The bad models contain models that are used for checking error messages. These can not be compiled by the stanc compiler.

`src/test/test-models`:
- `reference` has now been renamed `bad`
- `no-main` has now been moved to `good`
- `syntax-only` has also been moved to `good`


#### Intended Effect:

The old names don't mean much any more. This makes it easier to understand the layout of the test-models.

If we're adding more of these tests, it might make sense to simplify the make to be less optimal, but less error prone by requiring all the generated cpp files to be available to all the tests.

#### How to Verify:

Check the new layout.
All tests should pass. The tests haven't moved.

#### Side Effects:

None.

#### Documentation:

Not user facing. README.txt within test-models has been updated.

#### Reviewer Suggestions: 

@bob-carpenter. Now that you see it, iff you think of a better way to organize it, let me know.