cmd/go: test that 'go list -e -mod=readonly' reports errors correctly

This issue was fixed by earlier improvements to error handling when
loading modules.

Fixes #34829

Change-Id: I4cf4e182a7381f8b5c359179d90bd02491ea7911
Reviewed-on: https://go-review.googlesource.com/c/go/+/209037
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

+# 'go list -mod=readonly -e should attribute errors
+# to individual missing packages.
+# Verifies golang.org/issue/34829.
+go list -mod=readonly -e -deps -f '{{if .Error}}{{.ImportPath}}: {{.Error}}{{end}}' .
+stdout 'example.com/missing: use.go:3:8: import lookup disabled by -mod=readonly'
+
+-- go.mod --
+module example.com/m
+
+go 1.14
+
+-- use.go --
+package use
+
+import _ "example.com/missing"