cmd/go: fix TestScript/mod_sumdb_golang to avoid assumptions about @v/list

I accidentally fetched an invalid version of rsc.io/quote from
proxy.golang.org, which the proxy then cached and now includes in
https://proxy.golang.org/rsc.io/quote/@v/list.

That causes 'go get rsc.io/quote` to resolve to a different version
depending on whether the proxy is used.

Adjust the test to fetch an explicit version instead, since the choice
of 'latest' is mostly irrelevant to the checksum database logic that
the test is intended to verify.

Updates #32805
Fixes #32900

Change-Id: I075b1f62e8c71545d0fb2dd4bd77ba525fc2a36d
Reviewed-on: https://go-review.googlesource.com/c/go/+/184719
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/testdata/script/mod_sumdb_golang.txt

@@ -14,31 +14,39 @@ stdout '^sum.golang.org$'
 [!exec:git] skip
 env GOSUMDB=sum.golang.org
 env GOPROXY=direct
-go get -d rsc.io/quote
+go get -d rsc.io/quote@v1.5.2
+cp go.sum saved.sum
 
 # download from proxy.golang.org with go.sum entry already
 go clean -modcache
 env GOSUMDB=
 env GOPROXY=
-go get -x -d rsc.io/quote
+go get -x -d rsc.io/quote@v1.5.2
 ! stderr github
 stderr proxy.golang.org/rsc.io/quote
 ! stderr sum.golang.org/tile
 ! stderr sum.golang.org/lookup/rsc.io/quote
+cmp go.sum saved.sum
 
-# download again, using checksum database to validate new go.sum lines
+# Download again.
+# Should use the checksum database to validate new go.sum lines,
+# but not need to fetch any new data from the proxy.
 rm go.sum
-go get -x -d rsc.io/quote
+go get -x -d rsc.io/quote@v1.5.2
 ! stderr github
-stderr proxy.golang.org/rsc.io/quote
+! stderr proxy.golang.org/rsc.io/quote
 stderr sum.golang.org/tile
 stderr sum.golang.org/lookup/rsc.io/quote
+cmp go.sum saved.sum
 
 # test fallback to direct
 env TESTGOPROXY404=1
-go get -x -d rsc.io/quote
+go clean -modcache
+rm go.sum
+go get -x -d rsc.io/quote@v1.5.2
 stderr 'proxy.golang.org.*404 testing'
 stderr github.com/rsc
+cmp go.sum saved.sum
 
 -- go.mod --
 module m