use pytest-xdist to run the tests in parallel

Author: Vladimir Kotal

opengrok-tools/setup.cfg

@@ -2,5 +2,5 @@
 test=pytest
 
 [tool:pytest]
-addopts = -v --color=yes
+addopts = -v --color=yes --numprocesses=auto
 testpaths = src/test/python

opengrok-tools/setup.py

@@ -46,6 +46,7 @@ def readme():
     tests_require=[
         'pytest',
         'pygit2',
+        'pytest-xdist',
     ],
     entry_points={
         'console_scripts': [