cmd/go/internal/{modfetch,modload}: long tests fail when GOPROXY is set without GOSUMDB #31964
Description
~/go/src$ go version
go version devel +c8068ae1da Fri May 10 11:21:51 2019 -0400 linux/amd64
~/go/src$ GOSUMDB= GOPROXY=https://proxy.golang.org go test cmd/go/internal/{modfetch,modload} -count
=1
go: finding github.com/rsc/vgotest1 v0.0.0
go: finding github.com/rsc/vgotest1/submod v1.0.4
go: finding github.com/rsc/vgotest1 v1.1.0
2019/05/10 11:58:43 verifying github.com/rsc/vgotest1/submod@v1.0.4/go.mod: missing GOSUMDB
FAIL cmd/go/internal/modfetch 1.007s
go: finding golang.org/x/net latest
go: finding golang.org/x/net/context latest
go: downloading golang.org/x/net v0.0.0-20190509222800-a4d6f7feada5
2019/05/10 11:58:45 verifying golang.org/x/net@v0.0.0-20190509222800-a4d6f7feada5: missing GOSUMDB
FAIL cmd/go/internal/modload 2.269s
CC @jayconrod @rsc