Renamed 'udp_stub_connection.go' to 'udp_stub_connection_test.go'

Given that the UDP stubbing code in udp_stub_connection.go is only
being used by other testing code, it makes sense for the code to not be
included in production builds (by renaming the file to end with _test).

Furthermore, given that 'udp_stub_connection.go' imports the 'testing'
package, command-line flags were being implicitly added by calls to the
'flag' package from within the 'testing' package. This resulted in an
unnecessarily polluted flag namespace.

Ideally this stubbing code should be moved into its own library, but
for now this is a reasonable stop gap.