[lldb] Re-enable TestPlatformProcessConnect on macos

The test couldn't find lldb-server as it's path was being overridden by
LLDB_DEBUGSERVER_PATH environment variable (pointing to debugserver).
This test should always use lldb-server, as it tests its platform
capabilities.

There's no need for the environment override, as lldb-server tests
should test the executable they just built, so I just remote the
override capability.