CI: curl fallback, print killed tests, FreeBSD URL

- We've seen occasional 'ERROR 502: Bad Gateway' from the runner trying
to download an image with axel.  Axel can open multiple connections for
a faster download, so maybe that's causing problems.  This commit adds
in a fallback to curl if the axel download doesn't work.

- Update merge_summary.awk to print out killed tests in the summary.
We've seen cases where the summary page was red but there were no test
failures printed.  This is because one of the VMs had too may
killed tests, which caused the total test time to run too long and
caused the runner to timeout qemu-6-test.sh. When the runner kills off
qemu-6-tests.sh, it means we never generate the nice summary page
for that VM listing the killed off tests.  This commit parses the
partial test logs for killed off tests and includes them in the
merge_summary.awk output.

- Print an error message in the summary page if one of the VMs
didn't complete ZTS.  This helps draw attention to a VM crash.

- FreeBSD sometimes has broken links to their CI image. When that
happens, select the newest nightly snapshot image as an alternative.
This is needed right now, since the current images in the FreeBSD 16
"current/" directory are returning 404 errors.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #18460

.github/workflows/scripts/merge_summary.awk

@@ -17,6 +17,7 @@ BEGIN {
 	pass=0
 	fail=0
 	skip=0
+	killed=0
 	state=""
 	cl=0
 	el=0
@@ -49,6 +50,37 @@ BEGIN {
 /PASS/{ if (state=="pass_count") {pass += $2}}
 /FAIL/{ if (state=="pass_count") {fail += $2}}
 /SKIP/{ if (state=="pass_count") {skip += $2}}
+
+# If the test was killed, you'll get a line like:
+#
+# [2026-04-22T03:34:17.694616] Test (Linux): /usr/share/zfs/zfs-tests/tests/functional/io/setup (run as root) [10:00] [KILLED]
+#
+# Parse out the test name minus the /usr/share/zfs/zfs-tests/tests/functional/'
+# part, and include the optional "(Linux): " line, as you can have the killed
+# tests in two categories, like:
+#
+#   KILLED (Linux): io/setup
+#   KILLED io/setup
+#
+/KILLED/{
+	extra=""
+	for(i=1; i<=NF; i++) {
+		# Look for optional "(Linux):" field
+		if ($i ~ "\\("){
+			extra=$i" "}
+
+		# Look for a field with a '/' in it.  It is the test name.
+		if($i ~ "/") {
+			testname=$i
+			# Remove /usr/share/zfs/zfs-test/test/functional string
+			sub(/\/usr\/share\/zfs\/zfs-tests\/tests\/functional\//,"",testname)
+			testname=extra""testname
+			killed_tests[killed] = testname
+			killed++
+			break
+		}
+	}
+}
 /Running Time/{
 	state="";
 	running[i]=$3;
@@ -106,4 +138,10 @@ END {
 	asort(unexpected_lines, sorted)
 	for (j in sorted)
 		print sorted[j]
+
+	# We don't want to sort killed tests, as the first test that was killed
+	# most likely caused the others to be killed.
+	print "\n\nTests that were killed:"
+	for (j in killed_tests)
+		print "    KILLED "killed_tests[j]
 }