fix compile error (#7040)

Summary:
WITH_TESTS=OFF and WITH_BENCHMARK_TOOLS=ON

there has errors:
 /bin/ld: cannot find -ltestharness

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7040

Reviewed By: riversand963

Differential Revision: D22447637

Pulled By: ajkr

fbshipit-source-id: f63058376deb4a2e6722d63541c40caa617c331a

CMakeLists.txt

@@ -986,11 +986,14 @@ endif()
 
 option(WITH_ALL_TESTS "Build all test, rather than a small subset" ON)
 
-if(WITH_TESTS)
+if(WITH_TESTS OR WITH_BENCHMARK_TOOLS)
   add_subdirectory(third-party/gtest-1.8.1/fused-src/gtest)
   add_library(testharness STATIC
   test_util/testharness.cc)
   target_link_libraries(testharness gtest)
+endif()
+  
+if(WITH_TESTS)
   set(TESTS
         db/db_basic_test.cc
         env/env_basic_test.cc