src: check remained lines after all test cases

[Defined2014]

Fix https://github.com/pingcap/mysql-tester/issues/131, add a extra line in show.result file,

➜  integrationtest git:(master) ✗ git diff
diff --git a/tests/integrationtest/r/show.result b/tests/integrationtest/r/show.result
index 42232d8bea..410b1425f5 100644
--- a/tests/integrationtest/r/show.result
+++ b/tests/integrationtest/r/show.result
@@ -3,3 +3,4 @@ show tables like '%xx';
 Tables_in_show (%xx)
 show databases like '%xx';
 Database (%xx)
+aa

the result shown below

➜  integrationtest git:(master) ✗ ./run-tests.sh -t show
extracting statistics: s
skip building portgenerator, using existing binary: ./portgenerator
start tidb-server, log file: ./integration-test.out
tidb-server(PID: 28692) started
run integration test cases(enabled new collation): show
ERRO[0001] run test [show] err: There is extra data at the end of the result file: aa

ERRO[0001] 1 tests failed
ERRO[0001] run test [show] err: There is extra data at the end of the result file: aa

[Defined2014]

/merge