Improve error messages for common testsuite bugs.

Improve error messages for common testsuite bugs.

* lib/autotest/general.m4 (_AT_DEFINE_INIT, _AT_DEFINE_SETUP): New
macros for defining order-enforced macros.
(AT_INIT, AT_SETUP, AT_CLEANUP, AT_BANNER, AT_XFAIL_IF)
(AT_CAPTURE_FILE, AT_DATA, AT_CHECK, AT_CHECK_NOESCAPE): Add error
messages when order violations are detected.
* tests/autotest.at (AT_CHECK_AT_SYNTAX): New helper macro.
(AT_SETUP without AT_INIT, AT_BANNER without AT_INIT)
(AT_CLEANUP without AT_INIT, Missing AT_CLEANUP)
(AT_CHECK without AT_SETUP, AT_DATA without AT_SETUP)
(AT_XFAIL_IF without AT_DATA, AT_KEYWORDS without AT_SETUP,
(AT_CLEANUP without AT_SETUP, AT_BANNER inside AT_SETUP)
(AT_SETUP inside AT_SETUP, Multiple AT_INIT)
(Banner-only test suite): New tests.
Reported by Christopher Hulbert.

Signed-off-by: Eric Blake <ebb9@byu.net>