README: fix command for direct pytest invocation

Since a8f8f5e ("tests: split meson test() definitions into individual
test scripts"), we have needed to run pytest in the obj dir, but
reference the source tests/ tree.

Update the README to suit.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>

Author: jk-ozlabs

README.md

@@ -125,12 +125,12 @@ during development:
 
 ```sh
 cd obj
-pytest
+pytest ../tests
 ```
 
 To run without an existing dbus session:
 ```sh
-dbus-run-session env DBUS_STARTER_BUS_TYPE=user pytest
+dbus-run-session env DBUS_STARTER_BUS_TYPE=user pytest ../tests
 ```
 
 The test infrastructure depends on a few python packages, including the pytest