[lldb/test] Move `platform process list` tests to its own directory (NFC)

Since the `platform process` commamnd has more tests now, this commits
separates each of the `platform process` subcommand's test in its own directory.

Differential Revision: https://reviews.llvm.org/D74836



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