De-flake TestOCSP

[twifkak]

This TODO:

amppackager/packager/certcache/certcache_test.go

Lines 229 to 230 in bf061be

	// TODO(twifkak): Make this less flaky, by injecting a fake clock.
	this.Assert().Equal("public, max-age=302399", resp.Header.Get("Cache-Control"))

has become more important, now that we are running the test 5 times per PR (as of #424), so the chance of flaky failure is higher.