cmd/go,cmd/link: do not check for staleness in most tests
Instead, check that stale packages in the standard library
are not rebuilt when already present in the build cache,
and are not installed implicitly when rebuilt.
We retain the staleness checks for the runtime package in tests
involving '-i', because those are guaranteed to fail anyway if the
package is stale and the "stale" failure message is arguably clearer.
They can be removed if/when we remove the '-i' flag, but the runtime
package is less likely to become stale because it does not have cgo
dependencies.