Extended tests on AC_CONFIG_AUX_DIR.

Extended tests on AC_CONFIG_AUX_DIR.

* tests/auxdir.test: Enable `errexit' shell flag.  Prefer `$me'
over hard-coded test name.  Use proper m4 quoting.  Add trailing
`:' command.
* tests/auxdir2.test: Likewise.  Try to call automake also with
the `-a' option, so that it will not fail for spurious reasons.
* tests/auxdir3.test: Add an explanatory comment and a trailing
`:' command.
* tests/auxdir4.test: Prefer `$me' over hard-coded test name.
Make grepping of automake stderr slightly stricter.  Also, now
this test just checks about Automake's reaction to unportable
auxiliary directory names (and it has been extended in this
respect).  Moved the checks about non-existent auxiliary
directories to ...
* tests/auxdir5.test: ... this new test.
* tests/auxdir6.test: New test.
* tests/auxdir7.test: Likewise.
* tests/auxdir8.test: Likewise.
* tests/auxdir9.test: Likewise.
* tests/Makefile.am (TESTS): Updated.