bmcweb: add unit tests for processor FirmwareVersion handlers

Cover the callbacks introduced by the GPU Processor FirmwareVersion
change. The tests assert each branch's externally visible behavior:
unexpected D-Bus errors, an ambiguous subtree, and a missing owning
service set an internal server error, while EBADR, an empty subtree,
and an empty version leave FirmwareVersion omitted.

Change-Id: Ic00549fe7513651a139ebb8a4e7b36c95b3f453c
Signed-off-by: JY Voon <jvoon@nvidia.com>