Some tweaks to coverage support

Rename the configure option to --enable-coverage, and make
it quiet by default.
Test some gsettings corner case

Setting a strv to NULL was not tested before.
Improve test coverage for GPropertyAction
Slightly improve coverate for notification tests
contenttype tests: better assertions

I recently had to track down why these tests failed. Turned
out that some rogue package on my system had installed mime
types that declared all files with 3 letter names to be
'chemical/x-turbomole-vibrational'.
This change will make it more obvious what is going on by
mentioning the mime types in the assertion message.
Improve gsettings test coverage

Deal with corner cases around delay.
Fix a !srcdir problem with installed tests

When installing scripts which are not generated in a
srcdir != builddir build, $< will have ../.. goo in it.
So we need to strip the directory parts before using
the filename to construct a new path.