tests: don't clutter the top-level dir with temporary test directories

tests: don't clutter the top-level dir with temporary test directories

* t/self-check-me.tap: Be sure to initialize '$am_create_testdir' to "no"
in all the shell invocations sourcing './defs'.  Otherwise, when running
the testsuite with 'keep_testdirs=yes', the following temporary director
are left cluttering the top-level directory:

  ./012.dir
  ./abc..dir
  ./a.b.c.dir
  ./foo.dir
  ./foo.bar.dir
  ./foo-bar-.dir
  ./_foo__bar.dir

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>