cmd/go: add a regression test for 'go mod vendor' path traversal

For #46867

Change-Id: I1547ebf7b91e9ddd7b67fd2f20e91391d79fa35d
Reviewed-on: https://go-review.googlesource.com/c/go/+/332250
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

Author: Bryan C. Mills

src/cmd/go/testdata/script/mod_vendor_issue46867.txt

@@ -0,0 +1,31 @@
+# Regression test for golang.org/issue/46867:
+# 'go mod vendor' on Windows attempted to open and copy
+# files from directories outside of the module.
+
+cd subdir
+go mod vendor
+! exists vendor/example.net/NOTICE
+exists vendor/example.net/m/NOTICE
+
+-- subdir/go.mod --
+module golang.org/issue46867
+
+go 1.17
+
+replace example.net/m v0.1.0 => ./m
+
+require example.net/m v0.1.0
+-- subdir/issue.go --
+package issue
+
+import _ "example.net/m/n"
+-- subdir/m/go.mod --
+module example.net/m
+
+go 1.17
+-- subdir/m/n/n.go --
+package n
+-- subdir/m/NOTICE --
+This notice is in module m and SHOULD be vendored.
+-- subdir/NOTICE --
+This notice is outside of module m and SHOULD NOT be vendored.