selftests/namespace: commit_creds() active reference tests

Test credential changes and their impact on namespace active references.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-70-2e6f823ebdc0@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/namespaces/.gitignore

@@ -5,3 +5,4 @@ ns_active_ref_test
 listns_test
 listns_permissions_test
 siocgskns_test
+cred_change_test

tools/testing/selftests/namespaces/Makefile

@@ -8,12 +8,14 @@ TEST_GEN_PROGS := nsid_test \
 		  ns_active_ref_test \
 		  listns_test \
 		  listns_permissions_test \
-		  siocgskns_test
+		  siocgskns_test \
+		  cred_change_test
 
 include ../lib.mk
 
 $(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
 $(OUTPUT)/listns_test: ../filesystems/utils.c
 $(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
 $(OUTPUT)/siocgskns_test: ../filesystems/utils.c
+$(OUTPUT)/cred_change_test: ../filesystems/utils.c