Move test uniqueness check outside of _generate_test

This logic really belongs to the main loop, where the used_tests
variable, keeping track of the tests created so far, lives.

Bug: 330885075
Change-Id: I19e525132865abab2e84a5bc39ae0e2a42a3f7bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5374600


Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282135}