testsuite: move dg-test cleanup code from gcc-dg.exp to its own file

testsuite: move dg-test cleanup code from gcc-dg.exp to its own file

I need to use this cleanup logic for the testsuite for libdiagnostics
where it's too awkward to directly use gcc-dg.exp itself.

No functional change intended.

gcc/testsuite/ChangeLog:
	* lib/dg-test-cleanup.exp: New file, from material moved from
	lib/gcc-dg.exp.
	* lib/gcc-dg.exp: Add load_lib of dg-test-cleanup.exp.
	(cleanup-after-saved-dg-test): Move to lib/dg-test-cleanup.exp.
	(dg-test): Likewise for override.
	(initialize_prune_notes): Likewise.

libatomic/ChangeLog:
	* testsuite/lib/libatomic.exp: Add
	"load_gcc_lib dg-test-cleanup.exp".

libgomp/ChangeLog:
	* testsuite/lib/libgomp.exp: Add
	"load_gcc_lib dg-test-cleanup.exp".
libitm/ChangeLog:
	* testsuite/lib/libitm.exp: Add
	"load_gcc_lib dg-test-cleanup.exp".

libphobos/ChangeLog:
	* testsuite/lib/libphobos-dg.exp: Add
	"load_gcc_lib dg-test-cleanup.exp".

libstdc++-v3/ChangeLog:
	* testsuite/lib/libstdc++.exp: Add
	"load_gcc_lib dg-test-cleanup.exp".

libvtv/ChangeLog:
	* testsuite/lib/libvtv.exp: Add
	"load_gcc_lib dg-test-cleanup.exp".

Signed-off-by: David Malcolm <dmalcolm@redhat.com>