Run helloworld with `-tags=e2e` in presubmit (#1083)
Run helloworld with `-tags=e2e` in presubmit (#1083)

In #1053 we updated the e2e tests so they don't run unless `tags=e2e`
is specified, which means that when we just the hello world e2e test
we need to provide this flag or no actual tests will be run (though
they will quietly pass, outputing a "no test files" message).
knative/serving,knative/serving