Displaying file names when run parallel

Currently when tests are run on multiple cores the name of the files are not displayed any more. It would be rather useful to be able to still receive the same information even if it means one need to wait until all the end of the test run.

Without using numprocesses:

collected 114 items / 28 skipped / 86 selected                                                                                                                    

astropy/io/misc/yaml.py .                                                                                                                                   [  0%]
astropy/io/misc/tests/test_hdf5.py ........................................................ssss...........ss.                                               [ 65%]
astropy/io/misc/tests/test_pandas.py .....                                                                                                                  [ 70%]
astropy/io/misc/tests/test_pickle_helpers.py ....                                                                                                           [ 73%]
astropy/io/misc/tests/test_yaml.py ..............................                                                                                           [100%]

With numprocesses:

rootdir: /Users/bsipocz/munka/devel/astropy, inifile: setup.cfg
plugins: xdist-1.29.0, forked-1.0.2, arraydiff-0.3, cov-2.6.0, xonsh-0.8.3, dependency-0.4.0, doctestplus-0.3.0, requests-mock-1.6.0, openfiles-0.4.0.dev0, remotedata-0.3.2.dev0
gw0 C / gw1 C/usr/local/lib/python3.7/site-packages/pytest_doctestplus/plugin.py:10: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import imp
/usr/local/lib/python3.7/site-packages/pytest_doctestplus/plugin.py:10: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import imp
gw0 [114] / gw1 [114]1 skipped                                                                                                                                    
...................................................ssss.............s.s...........................................                                          [100%]

Also, having the info of the total number of collected and skipped tests is useful, too, and it's not printed in the latter case either.

Reproduce the two displayed runs with pytest-xdist and parallel workers, comparing serial output with the current worker output. Trace the xdist reporting and aggregation entry points, then verify that parallel runs retain file names plus collected and skipped totals without breaking the existing progress display.