[avalanche] Rationalize the options in the processor tests

Summary:
Consistently use the - prefix for the options (it works the same with or without it in ForceSetArg, but it gives a better example for users), and remove the duplicates.
There is no change in behavior.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13067