glib-macros: add test macro for async-glib-context tests #3889

Job	Run time
build GTK binaries with gvsbuild	13s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	1m 35s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	1m 50s
build gtk-rs on Windows (gio, true, --features v2_80)	2m 39s
build gtk-rs on Windows (glib, true, --features v2_80)	3m 57s
build gtk-rs on Windows (glib-build-tools, false)	2m 19s
build gtk-rs on Windows (graphene, false)	1m 44s
build gtk-rs on Windows (pango, true, --features v1_52)	2m 42s
build gtk-rs on Windows (pangocairo, true)	2m 17s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	2m 27s
	21m 43s

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