Expose forcibly_ggc_collect and run it after all selftests

Expose forcibly_ggc_collect and run it after all selftests

gcc/ChangeLog:
	* ggc-tests.c (forcibly_ggc_collect): Rename to...
	(selftest::forcibly_ggc_collect): ...this, and remove "static".
	(test_basic_struct): Update for above renaming.
	(test_length): Likewise.
	(test_union): Likewise.
	(test_finalization): Likewise.
	(test_deletable_global): Likewise.
	(test_inheritance): Likewise.
	(test_chain_next): Likewise.
	(test_user_struct): Likewise.
	(test_tree_marking): Likewise.
	* selftest-run-tests.c (selftest::run_tests): Call
	selftest::forcibly_ggc_collect at the end of the selftests.
	* selftest.h (selftest::forcibly_ggc_collect): New decl.

From-SVN: r241527