Miscellaneous test framework fixes

The issue fixed by commit b2dec895 can be reproduced by applying the following patch:

diff --git a/bin/named/os.c b/bin/named/os.c
index f413c71a095..0abfb6a0c6a 100644
--- a/bin/named/os.c
+++ b/bin/named/os.c
@@ -864,6 +864,7 @@ void
 named_os_shutdown(void) {
	closelog();
	cleanup_pidfile();
+	sleep(10);
	cleanup_lockfile();
 }

and running e.g. the unknown system test.

Example occurrences of the issue fixed by that commit include:

• https://gitlab.isc.org/isc-private/bind9/-/jobs/2224498
• https://gitlab.isc.org/isc-private/bind9/-/jobs/2253411

Also note that this commit will become redundant for main as soon as !5739 (merged) is merged, but it should still be reviewed because older maintenance branches are still going to need it.