Rename test_numerically to verify_numerically for py.test compatibility

Partial fix for #5641.  The function test_numerically was imported into many files to do numerical comparisons and resulted in the following error with py.test:

* On master
```bash
$ py.test sympy/simplify/tests/test_fu.py 
============================= test session starts ==============================
platform linux -- Python 3.4.1 -- py-1.4.20 -- pytest-2.5.2
architecture: 64-bit
cache:        yes
ground types: python 

collected 27 items 

sympy/simplify/tests/test_fu.py ..........................E

==================================== ERRORS ====================================
______________________ ERROR at setup of test_numerically ______________________
file /home/ptb/gitrepos/sympy/sympy/utilities/randtest.py, line 43
  def test_numerically(f, g, z=None, tol=1.0e-6, a=2, b=-1, c=3, d=1):
        fixture 'f' not found
        available fixtures: pytestconfig, recwarn, capsys, capfd, tmpdir, monkeypatch
        use 'py.test --fixtures [testpath]' for help on them.

/home/ptb/gitrepos/sympy/sympy/utilities/randtest.py:43
                                DO *NOT* COMMIT!                                
====================== 26 passed, 1 error in 2.71 seconds ======================
```

Renaming this function to something which doesn't start with 'test_' gets rid of the error

* This PR
```bash
$ py.test sympy/simplify/tests/test_fu.py 
============================= test session starts ==============================
platform linux -- Python 3.4.1 -- py-1.4.20 -- pytest-2.5.2
architecture: 64-bit
cache:        yes
ground types: python 

collected 26 items 

sympy/simplify/tests/test_fu.py ..........................

========================== 26 passed in 2.67 seconds ===========================
```