Print complement failure results last (#13639)

Since github always scrolls to the bottom of any test output, let's put the
failed tests last and hide any successful packages.

.ci/scripts/gotestfmt

@@ -0,0 +1,21 @@
+#!/bin/bash
+#
+# wraps `gotestfmt`, hiding output from successful packages unless
+# all tests passed.
+
+set -o pipefail
+set -e
+
+# tee the test results to a log, whilst also piping them into gotestfmt,
+# telling it to hide successful results, so that we can clearly see
+# unsuccessful results.
+tee complement.log | gotestfmt -hide successful-packages
+
+# gotestfmt will exit non-zero if there were any failures, so if we got to this
+# point, we must have had a successful result.
+echo "All tests successful; showing all test results"
+
+# Pipe the test results back through gotestfmt, showing all results.
+# The log file consists of JSON lines giving the test results, interspersed
+# with regular stdout lines (including reports of downloaded packages).
+grep '^{"Time":' complement.log | gotestfmt