[ios] Improve logic in ManualFillFormObserverHelperiOSTest

Improve logic in this test class to use EXPECT_OCMOCK_VERIFY instead of
try/catch blocks and update names to be consistent with our
guidelines.

Bug: none
Change-Id: I24ef7fe7f6bbfb0ffad96c24e3d08fe01dc0e7fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6225180
Auto-Submit: Federica Germinario <fedegermi@google.com>
Commit-Queue: Noémie St-Onge <noemies@google.com>
Reviewed-by: Noémie St-Onge <noemies@google.com>
Cr-Commit-Position: refs/heads/main@{#1414894}