On test failures, print a bash stack trace

This makes it easier to identify what command failed. It looks like:

  follow-paths.sh: test failed at:
    main in follow-paths.sh:54

tests/common.sh.in

@@ -190,4 +190,15 @@ if [[ -n "${NIX_DAEMON_PACKAGE:-}" ]]; then
     startDaemon
 fi
 
+onError() {
+    set +x
+    echo "$0: test failed at:" >&2
+    for ((i = 1; i < 16; i++)); do
+        if [[ -z ${BASH_SOURCE[i]} ]]; then break; fi
+        echo "  ${FUNCNAME[i]} in ${BASH_SOURCE[i]}:${BASH_LINENO[i-1]}" >&2
+    done
+}
+
+trap onError ERR
+
 fi # COMMON_SH_SOURCED