clear the cache at the start of file

On master, the cache is cleared after _every_ test when using pytest.  It is only cleared at the start of a testing file with `bin/test`.  This PR changes the behavior of pytest to match our native test runner.

* On master (with a print statement before the call to clear_cache)
```bash
sympy/assumptions/tests/test_assumptions_2.py .clearing cache
.clearing cache
.clearing cache
.clearing cache
.clearing cache

sympy/assumptions/tests/test_context.py .clearing cache
.clearing cache
.clearing cache
.clearing cache

sympy/assumptions/tests/test_matrices.py .clearing cache
.clearing cache
.clearing cache
xclearing cache
.clearing cache
.clearing cache
.clearing cache
.clearing cache
.clearing cache
.clearing cache
.clearing cache
.clearing cache
xclearing cache
.clearing cache
```

* This PR (with a print statement before the call to clear_cache)
```bash
sympy/assumptions/tests/test_assumptions_2.py clear cache
.....
sympy/assumptions/tests/test_context.py clear cache
....
sympy/assumptions/tests/test_matrices.py clear cache
...x........x......
sympy/assumptions/tests/test_query.py clear cache
...............x............x......
```