Modernize, improve and/or extend tests `colon*.test.

Modernize, improve and/or extend tests `colon*.test.

* tests/colon.test: Rely on the `configure.in' stub created by
`./defs', rather than writing one from scratch.  Do not create
a useless dummy file.  Add trailing `:' command.
* tests/colon4.test: Enable the `errexit' shell flag, and
related changes.  Rely on the `configure.in' stub created by
`./defs', rather than writing one from scratch.
* tests/colon7.test: Enable `errexit' shell flag, and related
changes.  Improve the generated `configure.in' file.  Add
trailing `:' command.
* tests/colon2.test: Likewise.  Also, add some new checks.
* tests/colon5.test: Improve the generated `configure.in' file.
Add new, much deeper checks.  Add trailing `:' command.
* tests/colon6.test: Likewise.
* tests/colon3.test: Improve the generated `configure.in' file.
Prefer perl over pipelined grep.  Made stricter.  Other changes,
cosmetic and not.