tests: fail with non-zero child exitcode/signal

Ref: neovim/lua-client#44

test/functional/helpers.lua

@@ -1,4 +1,5 @@
 require('coxpcall')
+local busted = require('busted')
 local luv = require('luv')
 local lfs = require('lfs')
 local mpack = require('mpack')
@@ -832,6 +833,16 @@ return function(after_each)
             error(msg[3][2])
           end
         end
+
+        if not session.child_exit then
+          session:close()
+        end
+        if session.child_exit then  -- requires newer neovim-lua-client (WIP).
+          if session.child_exit ~= 0 or session.child_signal ~= 0 then
+            busted.fail(string.format("child exited non-zero (exitcode=%d, signal=%d)",
+              session.child_exit, session.child_signal))
+          end
+        end
       end
     end)
   end