CSUB-1010: Fix Testnet Chainspec: make initial set invulnerable

[pLabarta]

Description of proposed changes

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Practical tips for PR review & merge:

• All GitHub Actions report PASS
• Newly added code/functions have unit tests
  • Coverage tools report all newly added lines as covered
  • The positive scenario is exercised
  • Negative scenarios are exercised, e.g. assert on all possible errors
  • Assert on events triggered if applicable
  • Assert on changes made to storage if applicable
• Modified behavior/functions - try to make sure above test items are covered
• Integration tests are added if applicable/needed

[atodorov]

Note to self: we can pull-in /~https://github.com/gluwa/creditcoin/blob/dev/integration-tests/src/test/danger-will-brick-the-blockchain.test.ts as part of this PR and/or do a bit more sanity checking. Will take another look into it once done with CSUB-978

[atodorov]

Tests & passing results in #171 (comment)

[zacharyfrederick]

Glad to see we are using the invulnerables setting. We should probably document it somewhere for the community. I would like to know that info.