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.

test/test-lib.sh

@@ -41,6 +41,10 @@ esac
 # Keep the original TERM for say_color and test_emacs
 ORIGINAL_TERM=$TERM
 
+# dtach(1) provides more capable terminal environment to anything
+# that requires more than dumb terminal...
+[ x"${TERM:-dumb}" = xdumb ] && DTACH_TERM=vt100 || DTACH_TERM=$TERM
+
 # For repeatability, reset the environment to known value.
 LANG=C
 LC_ALL=C
@@ -996,9 +1000,10 @@ test_emacs () {
 		fi
 		server_name="notmuch-test-suite-$$"
 		# start a detached session with an emacs server
-		# user's TERM is given to dtach which assumes a minimally
+		# user's TERM (or 'vt100' in case user's TERM is unset, empty
+		# or 'dumb') is given to dtach which assumes a minimally
 		# VT100-compatible terminal -- and emacs inherits that
-		TERM=$ORIGINAL_TERM dtach -n "$TEST_TMPDIR/emacs-dtach-socket.$$" \
+		TERM=$DTACH_TERM dtach -n "$TEST_TMPDIR/emacs-dtach-socket.$$" \
 			sh -c "stty rows 24 cols 80; exec '$TMP_DIRECTORY/run_emacs' \
 				--no-window-system \
 				$load_emacs_tests \