tests: Add a test for g_content_type_guess() with no arguments
tests: Add a test for g_content_type_guess() with no arguments

It should produce a generic result, but not crash. It was previously
crashing on macOS.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1729

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib