cmd/go: convert TestIssue6480 to the script framework

This one's a bit complex and required writing support go programs
within the test script.

Part of converting all tests to script framework to improve
test parallelism.

Updates #36320
Updates #17751

Change-Id: I9e91225b20b1b043f032b77a55c5825cb9d9a4b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/214292


Reviewed-by: Jay Conrod <jayconrod@google.com>