Test nxdtest via nxdtest

This allows to establish tests for Nxdtest itself under Nexedi testing
infrastructure.

/cc @jerome
/reviewed-on !4

.nxdtest

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