kpatch-test: add dmesg entry at start of test

If the kernel log is empty prior to running the integration tests, the
following confusing status may be reported:

  ...
  ERROR: dmesg overflow, try increasing kernel log buffer size
  SUCCESS

This occurs because the script can't find an empty dmesg entry when the
tests are complete.  Copy the upstream kernel livepatching kselftests to
fix this by logging a canary message at the beginning of the integration
tests.  This will ensure a "real" message than can be found at the end.

Fixes: de1d0c6 ("kpatch-test: don't clear dmesg during test")
Signed-off-by: Joe Lawrence <[email protected]>

test/integration/kpatch-test

@@ -299,12 +299,13 @@ run_combined_test() {
 
 # save existing dmesg so we can detect new content
 save_dmesg() {
-	SAVED_DMESG="$(dmesg | tail -n1)"
+	SAVED_DMESG="kpatch-test timestamp: $(date --rfc-3339=ns)"
+	echo "$SAVED_DMESG" > /dev/kmsg
 }
 
 # new dmesg entries since our saved entry
 new_dmesg() {
-	if ! dmesg | awk -v last="$SAVED_DMESG" 'p; $0 == last{p=1} END {exit !p}'; then
+	if ! dmesg --notime | awk -v last="$SAVED_DMESG" 'p; $0 == last{p=1} END {exit !p}'; then
 		error "dmesg overflow, try increasing kernel log buffer size"
 	fi
 }