Accelerate signer fixtures creation in tests

[sfauvel]

Content

Cryptographic operations takes a long time when we creating signers in tests.
To reduce this time, we precompute keskey for some party ids.

This PR stores in a file precomputed keskey for some party ids and use them if they match with the party id to create.
A test check that all keys are precomputed up to a party id number.
If precomputed keys are differents from the real ones, the test output the code to copy in function that provide precomputed keys.

Important

The directory /tmp/mithril_crypto_helper_material have to be removed to not reuse old kes.sk and opcert.cert files.

Time observed on the local environment before and after this PR.

	Aggregator	Signer	Common	Total
Before	52.6 s	5.7s	52.8s	111.1s
After	30.4s	3.9s	34.3s	68.7 s
Delta	-42%	-30%	-34%	-38%

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • No clippy warnings in the CI
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)

Issue(s)

Closes #2153

[github-actions]

Test Results

    4 files  ±0     52 suites  ±0   9m 45s ⏱️ - 2m 8s
1 429 tests +1  1 429 ✅ +1  0 💤 ±0  0 ❌ ±0 
1 640 runs  +1  1 640 ✅ +1  0 💤 ±0  0 ❌ ±0 

Results for commit 38314fe. ± Comparison against base commit f1441bc.

♻️ This comment has been updated with latest results.

[Alenar]

LGTM

[jpraynaud]

LGTM 👍

Can you add a brief summary about how much this accelerates the performances of the fixture creation?

[dlachaume]

LGTM 👍, just minor suggestions.