tests: Properly check if asserts are available

In my previous commits I failed to realise that my new requires lines fully
disabled these tests. We now properly check if we are in an asserts build and
only disable the tests if assertions are not available.

Reported-by: Sean Silva <silvas@purdue.edu>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@176900 91177308-0d34-0410-b5e6-96231b3b80d8