nmh unit test suite.

The purpose of these tests is to verify the functionality of the nmh
commands.  The goal of the suite is to create an environment where testing
nmh commands is easy and useful.  Each test is a shell script, and is
launched via the 'sh' command. The script should run the test and report
the result by one of:

  * for a test pass: exit with status 0
  * where a test has been skipped (perhaps because it depends on an
    external program which can't be found): print "Test $0 SKIP (reason)"
    and exit with status 77
  * for a test fail: exit with some status other than 0 or 120

The Suite is a re-worked version of the original test suite; it now is
designed to work with Automake.  To run these tests you can do "make check"
via the top-level Makefile.  This is also done automatically via
"make distcheck".

WARNING:  The test suite installs nmh and run the tests on that test
installation.  If you run tests individually, they will not remove
that test installation or check to see if it remains up to date with
your nmh workspace.  You can run test/clean after a test to remove the
test installation.  "make check" will do that, so it is best to use
it.

If you wish to write a new test script, here are the steps:

- Make sure your test script sources $MH_OBJ_DIR/test/common.sh and
  calls the setup_test shell function (the other scripts have examples
  of this).

- Your path will be set up to find the locations of the test nmh binaries.

- Add your script to the TESTS variable in the toplevel Makefile.am.
  By convention, test script names start with "test-", though that
  is not a requirement.

- If you need additional files for your tests, be sure to add them to
  the EXTRA_DIST variable in Makefile.am.  Note that you should insure
  that you access these files relative to the $srcdir environment variable.

- Verify that the test works with both "make check" and "make distcheck".