Fix harmless -Wconditional-uninitialized warnings in binder_test

This pedantic warning kicked in because the ASSERT() macros could
return from fakeRemoteSocketPair() without initializing all three
arguments. Clang is not sufficiently smart to see this.

So let's switch to unique_fd, which also results in slightly simpler
code...

Change-Id: I0c48ed4db7a3e808cd33b2a43a064871e3cdabeb