updating .gitignore to ignore generated .cpp files and not ignore .stan within test-models

#### Summary:

Fixes an issue with the .gitignore file.


#### Intended Effect:

New and changes to .stan files within src/test/test-models/* should be recognized by git.

#### How to Verify:

Add a .stan file to src/test/test-models/ and it should show up in git status. Also if you change a file in src/test/test-models/. If you create a .cpp file in one of the subdirectories, this should not show up in git status.

#### Side Effects:

No.

#### Documentation:

N/A.

#### Reviewer Suggestions:

@bob-carpenter: can you verify that this does the right thing?