[CMake] Only test debugserver if platform can use debugserver

In commit svn r351496 I changed this condition from
`if(LLDB_CAN_USE_DEBUGSERVER)` to `if(NOT SKIP_TEST_DEBUGSERVER)`.
This causes debugserver tests to run on windows, which shouldn't happen.
SKIP_TEST_DEBUGSERVER is set either by the user (it shouldn't be set on
windows builds) or in the debugserver CMake logic (which doesn't get
included when building on windows). Therefore, I changed the condition
to be `if(LLDB_CAN_USE_DEBUGSERVER AND NOT SKIP_TEST_DEBUGSERVER)`.

llvm-svn: 351498