Add a E2E test allowing to test validator set size changes

[lumtis]

Would be part of the admin test suite

The test send a proposal to decrease the validator set size and check network continue to run normally

We can also test with validator set size increase.

Right now we have 2 validators, reducing to 1 is already a sufficient test imo but to go further we can also considering an environment with more validators

[kingpinXD]

This test was already added in this pr https://github.com/zeta-chain/node/pull/3786/files#diff-a317fa4d370595fa975abfd95a858e4962f6a19775ab9e8247a0ea959ca79024