Add way to run tests via nxdtest

Nxdtest[1] is tox-like tool to run tests under Nexedi testing
infrastructure.

[1] https://lab.nexedi.com/nexedi/nxdtest

/cc @jerome, @gabriel
/reviewed-on !6

.nxdtest

+# setup to run tests on Nexedi testing infrastructure.
+# https://stack.nexedi.com/test_status
+
+TestCase('thread', ['python',  '-m', 'pytest'], summaryf=PyTest.summary)
+TestCase('gevent', ['gpython', '-m', 'pytest'], summaryf=PyTest.summary)