libtest-core: On failure, make it clearer what has happened

If we fail as a result of `set -x`, It's often not completely obvious
which command failed or how. Use a trap on ERR to show the command that
failed, and its exit status.

Signed-off-by: Simon McVittie <smcv@collabora.com>

tests/libtest-core.sh

@@ -162,3 +162,11 @@ skip() {
 extract_child_pid() {
     grep child-pid "$1" | sed "s/^.*: \([0-9]*\).*/\1/"
 }
+
+report_err () {
+  local exit_status="$?"
+  { { local BASH_XTRACEFD=3; } 2> /dev/null
+  echo "Unexpected nonzero exit status $exit_status while running: $BASH_COMMAND" >&2
+  } 3> /dev/null
+}
+trap report_err ERR