cmd/go: define how test caching interacts with timeouts

[rsc]

@rogpeppe suggests that -timeout simply be ignored when computing the test cache key.
See https://groups.google.com/d/msg/golang-dev/qfa3mHN4ZPA/9BNft8SFAQAJ.

We need to do something with -timeout, and that seems worth trying.

[gopherbot]

Change https://golang.org/cl/76552 mentions this issue: cmd/go: treat cached test results as satisfying any timeout