[lldb/test] Skip TestScriptedProcess when using system's debugserver (NFC)

Because TestScriptedProcess.py creates a skinny corefile to provides data
to the ScriptedProcess and ScriptedThread, we need to make sure that the
debugserver used is not out of tree, to ensure feature availability
between debugserver and lldb.

This also removes the `SKIP_SCRIPTED_PROCESS_LAUNCH` env variable after
each test finish running.

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>