ExitIfSmallMachine emits a helpful diagnostic and calls os.Exit(0) if the current machine is a builder known to have scarce resources.

NeedsGoPackages skips t if the go/packages driver (or 'go' tool) implied by the current process environment is not present in the path.

NeedsGoPackagesEnv skips t if the go/packages driver (or 'go' tool) implied by env is not present in the path.

NeedsTool skips t if the named tool is not present in the path.

Testing is an abstraction of a *testing.T.