Reapply "[lldb/test] Automatically find debug servers to test"

This reapplies 7df4eaaa/D96202, which was reverted due to issues on
windows. These were caused by problems in the computation of the liblldb
directory, which was fixed by D96779.

The original commit message was:
Our test configuration logic assumes that the tests can be run either
with debugserver or with lldb-server. This is not entirely correct,
since lldb server has two "personalities" (platform server and debug
server) and debugserver is only a replacement for the latter.

A consequence of this is that it's not possible to test the platform
behavior of lldb-server on macos, as it is not possible to get a hold of
the lldb-server binary.

One solution to that would be to duplicate the server configuration
logic to be able to specify both executables. However, that seems
excessively redundant.

A well-behaved lldb should be able to find the debug server on its own,
and testing lldb with a different (lldb-|debug)server does not seem very
us...