Fix GOPATH for platform tests check
Fix GOPATH for platform tests check

The platform tests vendor all their dependencies. We want this test to
ensure that they can be built using only thier vendored dependencies, so
we don't want to be adding anything else to the GOPATH for this case.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf