tools/lib/testutil: add "-short" to "go test" command in presubmit.
tools/lib/testutil: add "-short" to "go test" command in presubmit.

People can use Short() in their test to check whether this flag is set which we
will use to skip flaky tests.

(The end of the overview section of http://golang.org/pkg/testing/)

Change-Id: I040c5783a648a5ced7731497bf6b56e41f7e7c8d

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools