Dictation: Add SendFinalResultAndWaitForSelection()

This change merges three separate testing methods into one. This new
method takes a final speech result and an expected selection, and waits
until Dictation's internal selection state equals the provided one.

AX-Relnotes: N/A
Bug: b:259353252
Change-Id: If63dc53162d603a1c442a2e96474d8489553d445
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4189381
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1096927}