split out parser_test

#### Summary:
Splits out the parser_tests into appropriate subfiles and added missing parser tests for .stan files in test/test-models/syntax-only/ such as the assignment_... .stan files and rngs.stan.

#### Intended Effect:
Splits out the parser_tests into appropriate subfiles.

#### How to Verify:
Check test/unit/gm/parser/ tests.

#### Side Effects:
None.

#### Documentation:
None.

#### Reviewer Suggestions:
Anyone.