[lldb] Add import-std-module/sysroot to the libc++ test category.

We essentially test libc++ in a sysroot here so let's make sure
that we actually only run this test on platforms where libc++
testing is enabled.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@374572 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed