Only test sanitizers that are built when COMPILER_RT_SANITIZERS_TO_BUILD is used

Summary: This allows check-all to be used when only a subset of the sanitizers are built.

Reviewers: beanz, compnerd, rnk, pcc

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D34644

llvm-svn: 306450