cmd/go: move mod_get_svn test to vcs-test.golang.org

The test currently usses llvm.org, which seems to be very flaky today.

Change-Id: I3d01476d53f94d9170dbb087e3f3cf99581cdb4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/183847
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -2,12 +2,13 @@
 [!exec:svn] skip
 
 env GO111MODULE=on
-env GOPROXY=direct # obtain llvm.org directory, not via svn.
+env GOPROXY=direct
+env GOSUMDB=off
 
 # Attempting to get a module zip using svn should fail with a reasonable
 # message instead of a panic.
 # TODO(golang.org/issue/26092): Really, it shouldn't fail at all.
-! go get -d llvm.org/llvm/bindings/go/llvm
+! go get -d vcs-test.golang.org/svn/hello.svn
 stderr 'ReadZip not implemented for svn'
 ! go install .
 stderr 'ReadZip not implemented for svn'
@@ -16,5 +17,5 @@ stderr 'ReadZip not implemented for svn'
 module golang/go/issues/28943/main
 -- main.go --
 package main
-import _ "llvm.org/llvm/bindings/go/llvm"
+import _ "vcs-test.golang.org/svn/hello.svn"
 func main() {}