Strengthen cfi-check-fail test.

r258993 allows stricter testing for basic block labels by making
sure that they are always followed by ":". Use this to improve the
test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@258997 91177308-0d34-0410-b5e6-96231b3b80d8