build: Update glib-tap.mk from GLib
Use AM_TESTS_ENVIRONMENT rather than TESTS_ENVIRONMENT because the former is reserved for the user to set when running the tests; fix a misnamed variable; add missing mkdir for .test generation rule because it can’t be guaranteed that the builddir actually exists; and use TAP for installed tests too.
However, the Type is retained as 'session-exclusive' due to the presence of a graphical test.