REGTESTS: script: also add a line pointing to the log file

I never counted the number of hours I've been spending selecting then
copy-pasting the directory output and manually appending "/LOG" to read
a log file but it amounts in tens to hundreds. Let's just add a direct
pointer to the log file at the end of the log for a failed run.

scripts/run-regtests.sh

@@ -399,6 +399,7 @@ if [ -d "${TESTDIR}" ]; then
     cat <<- EOF | tee -a "$TESTDIR/failedtests.log"
 $(echo "###### $(cat "$i/INFO") ######")
 $(echo "## test results in: \"$i\"")
+$(echo "## test log file: $i/LOG")
 $(grep -E -- "^(----|\*    diag)" "$i/LOG")
 EOF
   done' sh {} +