Update to cockpit/pull/21233 testlib

This provides support for shadow DOM test selectors.

Makefile

@@ -33,7 +33,7 @@ COCKPIT_REPO_FILES = \
 	$(NULL)
 
 COCKPIT_REPO_URL = https://github.com/cockpit-project/cockpit.git
-COCKPIT_REPO_COMMIT = b0e82161b4afcb9f0a6fddd8ff94380e983b2238 # 328 + 23 commits
+COCKPIT_REPO_COMMIT = dc0d3e4a67d29b496d16bf5b8aa1ceb5d0778695 # https://github.com/cockpit-project/cockpit/pull/21233
 
 $(COCKPIT_REPO_FILES): $(COCKPIT_REPO_STAMP)
 COCKPIT_REPO_TREE = '$(strip $(COCKPIT_REPO_COMMIT))^{tree}'