Refactor tests for C

- Refactor `clang_tests` to be more extensible to other platforms
- add `grappalang_tests`
- run all tests with:
1. set GRAPPA_HOME
2. `python c_test_environment/grappalang_tests.py`
- run one specific test with:
`python c_test_environment/grappalang_tests.py GrappaTest.test_join`