tests: typo fix

tests/config/test.cfg

@@ -36,7 +36,7 @@ worker.coroutine(function ()
 	for idx, t in ipairs(tests) do
 		assert(type(t) == 'function',
 			string.format('test table idx %d in file %s'
-				.. 'is not a function', idx, env.TEST_FILE))
+				.. ' is not a function', idx, env.TEST_FILE))
 		at_least_one_test = true
 		runtest(t)
 	end