remove test dependency on compiling grammars

#### Summary:

Speeds up unit testing by only requiring the bin/libstanc.a when compiling grammars. This fixes issue #493.


#### Intended Effect:

Tests like:
```text
make test/unit/math/functions/step
make src/test/unit/meta
```

do not compile the grammars and are now very quick to compile and execute.


Tests like:
```text
make src/test/unit/gm
```
compiles the grammars and will take a long time to compile.


#### How to Verify:

This should run quickly: try any unit test folder excluding src/test/unit/gm. For example:
```
make src/test/unit/math/functions
```
That shouldn't compile the grammars.


This should compile the grammars and execute the tests:
```
make src/test/unit/gm
```


#### Side Effects:

None. Only changes to `make/tests`.


#### Documentation:

No public facing changes.


#### Reviewer Suggestions:

Bob.