tests: add testcases sanity-checking the testsuite

tests: add testcases sanity-checking the testsuite

Helper subroutines, variables and other pieces of code defined
in the `tests/defs' and used by many testcases are non-obvious,
and tricky to get to work portably; but until now, they weren't
tested at all in a clear and self-contained way.
This change should remedy to the situation.

* tests/self-check-cleanup.test: New test, check removal of
temporary test working directory by `./defs'.
* tests/self-check-dir.test: New test, check that tests using
`./defs' create a proper temporary directory, and run in it.
* tests/self-check-exit.test: New test, check that, in case of
failing commands, the correct exit status is passed to the exit
trap installed by the `./defs' script.
* tests/self-check-is_newest.test: New test, checking the
`is_newest' subroutine.
* tests/self-check-me.test: New test, checking that $me gets
defined automatically by `tests/defs' if not set, and that it
can be overridden from either the shell or the environment.
* tests/self-check-sanity.test: New test, check that the sanity
checks performed by the `tests/defs' script works correctly.
* tests/self-check-unindent.test: New test, checking the
`unindent' subroutine.
* tests/Makefile.am (TESTS): Update.