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/helpers.lua

@@ -6,7 +6,8 @@ local Session = require('nvim.session')
 
 local nvim_prog = os.getenv('NVIM_PROG') or 'build/bin/nvim'
 local nvim_argv = {nvim_prog, '-u', 'NONE', '-i', 'NONE', '-N',
-                   '--cmd', 'set shortmess+=I background=light', '--embed'}
+                   '--cmd', 'set shortmess+=I background=light noswapfile',
+                   '--embed'}
 local prepend_argv
 
 if os.getenv('VALGRIND') then
@@ -153,8 +154,7 @@ end
 
 local function clear()
   if session then
-    session:request('vim_command', 'qa!')
-    session:exit()
+    session:exit(0)
   end
   local loop = Loop.new()
   local msgpack_stream = MsgpackStream.new(loop)