libbinder: Move BinderRpc test fixture to new file

Move the BinderRpc test fixture and its dependencies
to a separate header file that can be included from
multiple source files, in preparation for splitting
the existing binderRpcTest tests into multiple sources.

Also split ProcessSession into an OS-independent base
class and an OS-specific derived class. Add a new
LinuxProcessSession class to binderRpcTest.cpp for the
latter.

Bug: 230135749
Test: atest binderRpcTest
Change-Id: I7a4464d5e210c716b6876ca2e16dc6f8748c6e9f