Print rerun counter 

When specify a large number for `--rerun` to test for failures it can be helpful to know how many reruns have already passed.  This PR prints the counter when --rerun is specified.  The rerun count is printed before the test.  Note that the counter is not printed for the zeroth run since it's technically not a rerun at that point.

Some examples.

* Regular behavior without `--rerun`
```bash
$ bin/test sympy/core/tests/test_cache.py
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
random seed:        34619550
hash randomization: on (PYTHONHASHSEED=542324729)

sympy/core/tests/test_cache.py[2] ..                                        [OK]

================== tests finished: 2 passed, in 0.01 seconds ===================
```

* New behavior with `--rerun`
```bash
$ bin/test sympy/core/tests/test_cache.py --rerun 3
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
random seed:        6013830
hash randomization: on (PYTHONHASHSEED=2232953711)

sympy/core/tests/test_cache.py[2] ..                                        [OK]

================== tests finished: 2 passed, in 0.01 seconds ===================
rerun 1
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
random seed:        53629875
hash randomization: on (PYTHONHASHSEED=2065740230)

sympy/core/tests/test_cache.py[2] ..                                        [OK]

================== tests finished: 2 passed, in 0.01 seconds ===================
rerun 2
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
random seed:        68537878
hash randomization: on (PYTHONHASHSEED=4011210339)

sympy/core/tests/test_cache.py[2] ..                                        [OK]

================== tests finished: 2 passed, in 0.01 seconds ===================
rerun 3
============================= test process starts ==============================
executable:         /home/ptb/miniconda3/bin/python  (3.4.1-final-0) [CPython]
architecture:       64-bit
cache:              yes
ground types:       python 
random seed:        51676033
hash randomization: on (PYTHONHASHSEED=1673038697)

sympy/core/tests/test_cache.py[2] ..                                        [OK]

================== tests finished: 2 passed, in 0.01 seconds ===================
```