tests: enable 'errexit' shell flag by default.
* tests/defs: Enable `errexit' shell flag (near the end).
Removed redundant comment about the enabling of shell traces.
* tests/README (Writing test cases): Update, and use nicer
formatting in a couple of places.
* All tests: Adjusted by removing now-redundant calls to
'set -e'.
New test on repeated automake options.
* tests/repeated-options.test: New test, check that automake
does not complain on repeated options, nor generate broken or
incorrect makefiles.
* tests/Makefile.am (TESTS): Updated.