This removes the gjs_coverage_new_for_cache() constructor. Instead,
gjs_coverage_new() always creates a cache in the default location, since
it is unlikely that you would ever want to run coverage without a cache
except in GjsCoverage's unit tests.

For the tests, this adds two new constructors to coverage-internal.h,
_gjs_coverage_new_internal_with_cache() and
_gjs_coverage_new_internal_without_cache().

As for creating a GjsCoverage object directly with g_object_new(), you
can unfortunately still get whatever behaviour you want from the public
API. If you don't specify a "cache" property at construct time, you get
the default cache. If you do specify it and it is NULL, you get no cache.

https://bugzilla.gnome.org/show_bug.cgi?id=775776