Show which notmuch command and version is being used

This add the notmuch version and absolute path of the binary used
in the pytest header.  This is nice when running the tests
interactively as you get confirmation you're testing the version you
thought you were testing.

bindings/python-cffi/tests/conftest.py

@@ -10,6 +10,13 @@ import os
 import pytest
 
 
+def pytest_report_header():
+    vers = subprocess.run(['notmuch', '--version'], stdout=subprocess.PIPE)
+    which = subprocess.run(['which', 'notmuch'], stdout=subprocess.PIPE)
+    return ['{} ({})'.format(vers.stdout.decode(errors='replace').strip(),
+                             which.stdout.decode(errors='replace').strip())]
+
+
 @pytest.fixture(scope='function')
 def tmppath(tmpdir):
     """The tmpdir fixture wrapped in pathlib.Path."""