tap: non-zero exit status after "Bail out!" should not be reported

tap: non-zero exit status after "Bail out!" should not be reported

This new behaviour is more consistent with that of the `prove'
utility.  Also, experience with the use of TAP in the Automake's
own test suite has shown that reporting non-zero exit status from
a script after it has issued a "Bail out!" directive is mostly
redundant, more introducing noise rather than conveying useful
information.

* lib/tap-driver (finish): Inline the part of it that was getting
the script exit status ...
(main): ... in here.
* tests/tap-bailout-badexit.test: New test.
* tests/Makefile.am (tap_with_common_setup_tests): Update.