[js/web] update test to explicitly fail for webnn without proxy (#17554)
### Description
Update test to explicitly fail for webnn without proxy.
I am doing this change because if I test webnn with other backend
together, it silently enables proxy. I want to make test runner behave
with less implicit flag reset. If proxy is not enabled, webnn test
should fail.
@Honry please let me know if other places (eg. CI scripts) should change
also.