Add test case for using an udev monitor with a filter

Side issue from https://launchpad.net/bugs/1336671
Add umockdev_testbed_get_property() method

This is mostly for internal umockdev testing, umockdev users should use
libudev/gudev get_property functions.
preload: Make path redirection to testbed thread-safe for wrapped functions

This does NOT address all multi-thread issues, but should work for simple
cases.

https://launchpad.net/bugs/1336671