cros-preview: Refactor useTestDelay function name

- Update handler function name to be 'setTestDelay' instead of 'useTestDelay'.

Bug: b:323421684
Test: browser_test --gtest_filter=*PrintPreviewCros*
Change-Id: I950883180cc7f9361b33a2894bc0f082da7de491
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5422511


Commit-Queue: Ashley Prasad <ashleydp@google.com>
Reviewed-by: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283197}