x/pkgsite: kokoro presubmit test is flaky

[hyangah]

observed during go.dev/cl/483757

https://source.cloud.google.com/results/invocations/c9abece0-cec4-4ca0-b03b-48e55f00b2ab/targets/golang%2Fpkgsite%2Fgcp_ubuntu%2Fpresubmit/log

2023/04/12 18:39:56 Critical: fetchFunc(ctx, f, "std", "v1.9.0"): FetchAndUpdateState("std", "v1.9.0", ""): fetchAndInsertModule("std", "v1.9.0"): FetchModule("std", "v1.9.0"): stdlib.ZipInfo("v1.9.0"): semanticVersion("v1.9.0"): stdlib.Versions(): remoteGoRepo.refs: Get "https://go.googlesource.com/go/info/refs?service=git-upload-pack": context deadline exceeded
exit status 1

https://source.cloud.google.com/results/invocations/1e04a480-457f-4f52-a2cf-1167dd4ba62f/targets/golang%2Fpkgsite%2Fgcp_ubuntu%2Fpresubmit/log
Similar timeout issue during screentest.

https://source.cloud.google.com/results/invocations/766b4c7f-c19c-4ed2-8726-d37da0c27b61/targets/golang%2Fpkgsite%2Fgcp_ubuntu%2Fpresubmit/log

2023/04/12 19:22:10 Debug: test-1549 DROP TABLE IF EXISTS ser args=
2023/04/12 19:22:10 Error: {ID:test-1549 Query:DROP TABLE IF EXISTS ser Args: DurationSeconds:9.443e-06 Error:DB running query test-1549: context deadline exceeded}
--- FAIL: TestTransactSerializable (10.01s)
database_test.go:433: max retries: 0

[gopherbot]

Change https://go.dev/cl/484738 mentions this issue: internal/fetchdatasource: remove arbitrary test timeout

[hyangah]

From some of the kokoro failure logs, I observed the system does not only fetch known canonical versions, but also attempts to fetch /@v/main.info and /@v/master.info. (see Fetcher.fetchAndInsertModule). This can
slow down the fetch further - proxy will try to fetch from vcs to serve the latest version @master or @main.

2023/05/02 14:36:44 Info: Fetch requested: "[github.com/hashicorp/vault/api](https://www.google.com/url?q=http://github.com/hashicorp/vault/api&sa=D)" "v1.1.1"
2023/05/02 14:36:44 Warning (map[fetch:[github.com/hashicorp/vault/api@v1.1.1]](https://www.google.com/url?q=http://github.com/hashicorp/vault/api@v1.1.1%5D&sa=D)): fetch.GetInfo(ctx, [github.com/hashicorp/vault/api](https://www.google.com/url?q=http://github.com/hashicorp/vault/api&sa=D), "main", getter, false): [proxy.Client.Info](https://www.google.com/url?q=http://proxy.Client.Info&sa=D)("[github.com/hashicorp/vault/api](https://www.google.com/url?q=http://github.com/hashicorp/vault/api&sa=D)", "main"): Client.readBody("[github.com/hashicorp/vault/api](https://www.google.com/url?q=http://github.com/hashicorp/vault/api&sa=D)", "main", "info"): executeRequest(ctx, "[https://proxy.golang.org/github.com/hashicorp/vault/api/@v/main.info](https://www.google.com/url?q=https://proxy.golang.org/github.com/hashicorp/vault/api/@v/main.info&sa=D)"): "not found: fetch timed out": proxy timed out
...
2023/05/02 14:36:49 Info (map[fetch:[github.com/tendermint/tendermint@v0.34.10]](https://www.google.com/url?q=http://github.com/tendermint/tendermint@v0.34.10%5D&sa=D)): deleted 0 rows from symbol_search_documents
2023/05/02 14:36:50 Info (map[fetch:[github.com/tendermint/tendermint@v0.34.10]](https://www.google.com/url?q=http://github.com/tendermint/tendermint@v0.34.10%5D&sa=D)): Updated module version state for [github.com/tendermint/tendermint@v0.34.10](https://www.google.com/url?q=http://github.com/tendermint/tendermint@v0.34.10&sa=D): code=290, num_packages=153, err=<nil>; timings: db.InsertModule=8.509s, db.UpdateModuleVersionState=0.017s, fetch.FetchModule=4.065s, worker.updatedVersionMap=0.001s
2023/05/02 14:36:50 Info: Fetch requested: "[github.com/tendermint/tendermint](https://www.google.com/url?q=http://github.com/tendermint/tendermint&sa=D)" "v0.34.11"
2023/05/02 14:36:50 Warning (map[fetch:[github.com/tendermint/tendermint@v0.34.11]](https://www.google.com/url?q=http://github.com/tendermint/tendermint@v0.34.11%5D&sa=D)): fetch.GetInfo(ctx, [github.com/tendermint/tendermint](https://www.google.com/url?q=http://github.com/tendermint/tendermint&sa=D), "master", getter, false): [proxy.Client.Info](https://www.google.com/url?q=http://proxy.Client.Info&sa=D)("[github.com/tendermint/tendermint](https://www.google.com/url?q=http://github.com/tendermint/tendermint&sa=D)", "master"): Client.readBody("[github.com/tendermint/tendermint](https://www.google.com/url?q=http://github.com/tendermint/tendermint&sa=D)", "master", "info"): executeRequest(ctx, "[https://proxy.golang.org/github.com/tendermint/tendermint/@v/master.info](https://www.google.com/url?q=https://proxy.golang.org/github.com/tendermint/tendermint/@v/master.info&sa=D)"): "not found: fetch timed out": proxy timed out

Does the screentest depend on the master and main results too? If not, I wonder disabling the fetch (e.g. internal/proxy.Client.WithFetchDisabled) may help reducing the test time. Currently the test often fails with context deadline exceeded (5min deadline).