Ensure pytest skips tests that have been disabled

`bin/test` tests if required modules are available for certain tests and if not does not run the test.  Some of these files had corresponding `setup_module` functions so pytest would skip them if the required modules were not available and some did not.  This PR ensures that `bin/test` and `pytest` will see the same tests as disabled/skipped

* On master (no numpy installed)
```bash
$ py.test sympy/plotting/intervalmath/tests/test_interval_functions.py 
============================= test session starts ==============================
platform linux2 -- Python 2.7.8 -- py-1.4.20 -- pytest-2.5.2
architecture: 64-bit
cache:        yes
ground types: python 

collected 24 items 

sympy/plotting/intervalmath/tests/test_interval_functions.py FFFFFFFFF..FFFFFFFFFF...
```

* PR (still no numpy)
```bash
$ py.test sympy/plotting/intervalmath/tests/test_interval_functions.py 
============================= test session starts ==============================
platform linux2 -- Python 2.7.8 -- py-1.4.20 -- pytest-2.5.2
architecture: 64-bit
cache:        yes
ground types: python 

collected 24 items 

sympy/plotting/intervalmath/tests/test_interval_functions.py ssssssssssssssssssssssss

========================== 24 skipped in 0.22 seconds ==========================
```