cmd/go: eliminate test dependencies on cloud.google.com/go

[bcmills]

CL 196961 added a test that fetched a module from cloud.google.com/go.

Unfortunately, the rate of 5XX-series errors for fetches from that repo has increased substantially since around 2021-11-04, quite possibly due to the same underlying problem as #50541.

Since the cloud.google.com server isn't controlled by the Go project proper, we should probably eliminate this test dependency.
(CC @matloob)

greplogs --dashboard -md -l -e 'https://cloud\.google\.com/go.*: 5\d\d\b'

2022-02-04T23:42:58-7c9885d/linux-386-longtest
2022-01-25T00:39:08-16d6a52/linux-386-longtest
2022-01-20T19:24:26-2c2e081/linux-386-longtest
2022-01-17T09:23:25-897b3da/linux-386-longtest
2021-12-21T18:34:03-cfb0cc3/linux-amd64-longtest
2021-12-20T23:46:23-1555062/linux-amd64-longtest
2021-11-16T06:36:08-a52e4b9/linux-386-longtest
2021-11-13T02:30:25-c546052/linux-386-longtest
2021-11-04T17:07:48-5772877/linux-amd64-longtest
2021-11-04T16:59:45-6e7b82a/linux-386-longtest
2021-08-16T20:22:08-4466141/linux-386-longtest
2020-07-09T16:26:07-72735e7/linux-386-longtest

[bcmills]

https://build.golang.org/log/4817f8416202d679733c88b235c161ccbe8d7817 provides fairly compelling evidence that this is, in fact, related to #50541: in that log, a cloud.google.com/go fetch with a 500 error occurred in close correspondence with a go.googlesource.com fetch with a 502.

[bcmills]

greplogs -l -e 'https://cloud\.google\.com/go.*: 5\d\d\b' --since=2022-02-05
2022-05-04T00:51:08-8b389eb/linux-386-longtest
2022-04-25T14:52:51-6b1d9ae/linux-386-longtest
2022-03-01T19:46:03-0fcb948/linux-386-longtest
2022-02-08T23:38:05-0b7e586/linux-386-longtest
2022-02-08T22:25:54-5406328/linux-386-longtest