Added --exe argument to nosetest default

See https://github.com/iterative/dvc/issues/1736#issuecomment-473665122

tests/__main__.py

@@ -21,6 +21,10 @@ cmd = (
     "nosetests -v --processes=4 --process-timeout=600 --cover-inclusive "
     "--cover-erase --cover-package=dvc --with-coverage --with-flaky "
     "--logging-clear-handlers "
+    # --exe: ensures tests are executed even if `basic_env.py` has umask 0777
+    #   and owned by root:root as it does when running in Windows Subsystem for
+    #   Linux (WSL).
+    "--exe "
     "{scope} ".format(scope=scope)
 )
 check_call(cmd, shell=True)