Improve and extend tests on `:=' variable assignments.

Improve and extend tests on `:=' variable assignments.

* tests/colneq.test: Avoid redundant use of variable assignments
in Makefile.am.  Use command-line automake options instead of
editing AUTOMAKE_OPTIONS in Makefile.am.  Make grepping of the
generated Makefile.in slightly stricter.  Add a trailing `:'
command.
* tests/colneq2.test: Do not create unneeded dummy files.  Run
also autoconf, ./configure and make.  Add trailing `:' command.
* tests/colneq3.test: New test, similar to colneq.test, but
running also autoconf, ./configure and make.
* tests/Makefile.am (TESTS): Update.