test/test-lib.sh: use vt100 as dtach terminal if TERM dumb or unset/empty

The TERM environment variable is set to 'dumb' when running tests, but
the original value of it is stored for echoing colors and running emacs
(somewhat interactively) in detached session. Emacs requires some
terminal control sequences to be available for interactive operation.
In case original TERM is (also) 'dumb' (or unset/empty) emacs cannot
run interactively. To fix this problem dtach (and emacs as it's child
process) is run with TERM=vt100 in case original TERM was unset, empty
or 'dumb'. This way there is a chance to run emacs tests with different
user terminals and potentially find problems there.