Use pytest as default test runner instead of unittest for tox

There was some trouble getting this to work. I needed to use
--pyargs for pytest to work in tox

Allow for test file prefix unittest_ to be changed to standard test_
Helps with updating the astroid tests to standard pytest conventions

Make pytest use proper test directory by default

setup.cfg

 [wheel]
 universal = 1
 [tool:pytest]
-python_files = unittest_*
+python_files = unittest_* test_*.py
+testpaths = astroid/tests

tox.ini

@@ -27,7 +27,11 @@ setenv =
     COVERAGE_FILE = {toxinidir}/.coverage.{envname}
 commands =
-    python -Wi -m coverage run -m unittest {posargs: discover -s astroid/tests -p "unittest*.py"}
+    ; --pyargs is needed so the directory astroid doesn't shadow the tox
+    ; installed astroid package
+    ; This is important for tests' test data which create files
+    ; inside the package
+    python -Wi {envsitepackagesdir}/coverage run -m pytest --pyargs astroid {posargs:}
     ; Transform absolute path to relative path
     ; for compatibility with coveralls.io and fix 'source not available' error.
     ; If you can find a cleaner way is welcome