test: add functional tests for coinjoinsalt RPC

[kwvg]

Additional Information

• Current suite of tests do not check if restoring salt results in restoring CoinJoin balance. This is because functional tests currently do not test CoinJoin mixing (and thus, routines for the same are not currently present).

Breaking Changes

None expected.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation (note: N/A)
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[knst]

LGTM overall 3106add

just a note:
if #6479 will get merged first, need to update this PR to remove coinjoinsalt from list of exception; if this PR get merged first I need to update 6479

UPD: 6479 got merged, please remove an exception for coinjoinsalt

[knst]

utACK 16c2e13

[UdjinM6]

utACK 16c2e13