Fix Binding builder to only use ref

The testbuilder `EventListenerTriggerBinding` allows users to specify both
`name` and `ref` fields. With #617, specifying both `name` and `ref` is no
longer allowed.

We fixed the builder usage for all the happy path tests to use `name` as an
empty string but did not do it for tests that are verifying error paths (such
as validation errors). So, these tests will now all fail at the same validation
check making most of these tests useless (causing the drop in coverage in

To fix, I changed the builder to only accept `ref` so that one cannot create
invalid structs. Later, we can migrate our tests to not use builders at all and
have our error tests verify the actual error string.

Signed-off-by: Dibyo Mukherjee <dibyo@google.com>

Author: dibyom