In inttest/proto_gpb, replace the external dependency to gpb, a
protocol buffer compiler, with a simple mock.
The mock provides enough of what is needed for inttest only,
which is to generate some dummy content, but with proper file names.
As a result, the increased timeout for the proto_gpb inttest should no
longer be needed (inttest/proto_gpb/retest.config), both since the
mock is now much faster to compile, and since any network bandwidth or
connectivity issues have now been eliminated.
