Some support for debug builds

Now you can do `make BUILD_OPTIONS=debug`.
Debug printouts for refcounting
Disable gdbus/gvfs usage when running tests

The gvfsdbus backend adds a lot of runtime state and activity
highly dependent on local configuration. Even if it may reveal
problems in our event loop code, the added noise and lack of
predictability is unwelcome for testing and the debug tools.
Added an ignored test for off-stack calls

This test crashes, and running tests in parallel results in
a hang-up, possibly due to a race in g_type_init().
Serialize the test run

It's pretty bad form that grust::init() is not reentrant.
But even though that should be fixed, running tests in parallel
creates unpredictability in on-stack vs. off-stack paths that
contest the default GMainContext. I should think about isolating
the tests better, as surely parallel test runs are a good stress
test for thread safety.
Added a couple more tests that don't crash