test: Remove option to make TestChain100Setup non-deterministic

Summary:
Seems odd to have an option for non-deterministic tests
when the goal should be for all tests to be deterministic.

Can be reviewed with --ignore-all-space.

This concludes backport of core#21592
https://github.com/bitcoin/bitcoin/pull/21592/commits/fa6183d7761eef8bb815aa888f0396e92e275f05

Notes:

• finalization_tests assumed that the mocktime was later than the node startup time, because the finalization delay is check both against the startup time and current time
• the 2 avalanche tests worked because the blocks mined by the test setup were all mined with the same block time, so using the MTP of the tip as base time for following blocks worked. Now the MTP of the base block is 6 seconds behind the tip, so we need to use the time of the tip to achieve a final MTP that actually expires the proof.

Test Plan: ninja check

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12309