webnn: add more tests for argMin/argMax ops

This cl adds more tests for outputDataType option.
This cl also adds fp16 conformance tests.

Bug: 354626180, 345280754
Change-Id: I5932270910f8cb2ae85b3b196723bec7233133bf
Cq-Include-Trybots: luci.chromium.try:win11-blink-rel, mac14.arm64-blink-rel, mac15.arm64-blink-rel, linux-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6034949
Reviewed-by: ningxin hu <[email protected]>
Reviewed-by: Austin Sullivan <[email protected]>
Auto-Submit: Feng Dai <[email protected]>
Commit-Queue: Feng Dai <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1410098}