[OpenMP][AMDGPU] Enable omptest build

This enables building the omptest library across the AMD buildbots that
rely on this CMake cache.

Author: jplehr

offload/cmake/caches/AMDGPUBot.cmake

@@ -17,6 +17,8 @@ set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
 set(LLVM_TARGETS_TO_BUILD "host;AMDGPU;SPIRV" CACHE STRING "")
 set(LLVM_LIT_ARGS "-v --show-unsupported --timeout 100 --show-xfail -j 16" CACHE STRING "")
 
+set(LIBOMPTEST_BUILD_UNITTESTS ON CACHE BOOL "")
+
 set(CLANG_DEFAULT_LINKER "lld" CACHE STRING "")
 set(CLANG_DEFAULT_RTLIB "compiler-rt" STRING "")