cmake: make GTEST_PARALLEL_COMMAND visible to all test directories

cmake: make GTEST_PARALLEL_COMMAND visible to all test directories

GTEST_PARALLEL_COMMAND was set as an ordinary variable inside the
`if(NOT TARGET gtest-parallel_ext)` guard, so it only existed in the
first directory that include()s AddCephTest (src/common/options). Later
includes skip the guarded block and leave it empty, so PARALLEL
unittests under src/test silently ran serially.

Promote it to CACHE INTERNAL so it is visible across all directories
regardless of include order.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>