[test] Build sanitizer/xray tests only if COMPILER_RT_BUILD_* is on

Cover the sanitizer tests with COMPILER_RT_BUILD_SANITIZERS
conditional, and add COMPILER_RT_BUILD_XRAY conditional to the xray
tests. This makes it possible to do a pure-builtins build with tests
enabled.

Differential Revision: https://reviews.llvm.org/D32489

llvm-svn: 301387