Ignore g_return_*if_fail() branches in lcov coverage report

Ignore precondition assertions, g_assert_if_reached(), and all the branching in test assertions like g_assert_cmpstr(). This removes a lot of false negatives from our code coverage.

Sadly there seems to be no way to make it generic across all projects which use GLib — they’ll all have to use their own copy of our lcovrc. See https://github.com/linux-test-project/lcov/issues/44.