Fix py.test failure in test_coordinate_vars

Add fixture so py.test can inject variables into function global namespace.  Fixes failures in test_coordinate_vars

* On Master
```bash
$ py.test sympy -k test_coordinate_vars
============================= test session starts ==============================
platform linux -- Python 3.4.1 -- py-1.4.25 -- pytest-2.6.3
architecture: 64-bit
cache:        yes
ground types: python 

collected 6406 items 

sympy/physics/vector/tests/test_frame.py F
sympy/vector/tests/test_coordsysrect.py F

=================================== FAILURES ===================================
_____________________________ test_coordinate_vars _____________________________

    def test_coordinate_vars():
        """Tests the coordinate variables functionality"""
>       assert CoordinateSym('Ax', A, 0) == A[0]
E       assert Ax == A_x
E        +  where Ax = CoordinateSym('Ax', A, 0)

sympy/physics/vector/tests/test_frame.py:12: AssertionError
_____________________________ test_coordinate_vars _____________________________

    def test_coordinate_vars():
        """
        Tests the coordinate variables functionality with respect to
        reorientation of coordinate systems.
        """
>       assert BaseScalar('Ax', 0, A, ' ', ' ') == A.x
E       assert Ax == A.x
E        +  where Ax = BaseScalar('Ax', 0, A, ' ', ' ')
E        +  and   A.x = A.x

sympy/vector/tests/test_coordsysrect.py:63: AssertionError
                                DO *NOT* COMMIT!                                
============== 6404 tests deselected by '-ktest_coordinate_vars' ===============
============ 2 failed, 6404 deselected, 1 warnings in 22.61 seconds ============
```

* This PR
```bash
$ py.test sympy -k test_coordinate_vars
============================= test session starts ==============================
platform linux -- Python 3.4.1 -- py-1.4.25 -- pytest-2.6.3
architecture: 64-bit
cache:        yes
ground types: python 

collected 6406 items 

sympy/physics/vector/tests/test_frame.py .
sympy/vector/tests/test_coordsysrect.py .

============== 6404 tests deselected by '-ktest_coordinate_vars' ===============
============ 2 passed, 6404 deselected, 1 warnings in 13.91 seconds ============
```