testsuite: factor out fd related parameters to a structure

This is a more abstract implementation of "file descriptor
comparation". With the current implementation the code is full of
conditions based on the descriptor type. It makes sense to
initialize the parameters once based on the descriptor type.

stdout and stderr are handled in almost the same way, but for
monitor descriptor branch, based on the type check is necessary in
some cases.

Since epoll's context now contains pointers to the structures, so no
direct manipulations there.

Most of the patch is just replacing direct buffer manipulations with
the structures' ones.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>