test: Fix hanging test suite after failures

When a test that fails leaves nvim in a 'Press Enter...' state, the whole suite
will hang because the `qa!` command executed before the next test won't be
processed until '<enter>' is sent.

Now the lua client can send a signal with when `Session:exit()` is called, so
the `qa!` request is no longer necessary.

Also:

- Set noswapfile at startup to prevent tests from leaving .s* swap files(should
  also improve test environment determinism)
- Use `assert(false, msg) instead of `error(msg)` to report screen assertion
  failures.

test/functional/ui/screen.lua

@@ -192,7 +192,7 @@ function Screen:wait(check, timeout)
     err = check()
   end
   if err then
-    error(err)
+    assert(false, err)
   end
 end