glib/tests/meson.build: remove identical build targets

This is a follow up of the discussion in !3590 (merged)

If no error with -Werror=sign-conversion, the resulting object file does not differ from compilation without -Werror=sign-conversion. So the -Werror argument is now applied directly to string.c and testing.c. Finally, the currently specific -Werror targets string-macro and testing-macro are removed.