testsuite harness: report test exit status in log file

testsuite harness: report test exit status in log file

The exit status of a test should be reported in the test logs, so
that one can see at a glance whether the test has succeeded or failed,
without having to look also into the corresponding .trs file.

This fixes automake bug#11814.

* lib/test-driver: Also report the test script exit status in the
test log (as the last line).
* t/check-exit-status-reported.sh: Test this new behaviour.
* t/list-of-tests.mk: Add the new test.
* t/ax/test-lib.sh( am_exit_trap): No longer log the test exit status;
this has been made redundant by the change to 'test-driver'.  While at
it, fix an imperfect quoting.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>