chore: replaced friend for unit-test with mock

[vlntb]

Improving unit-test that have to access class internals

Context of Change

Replacing friend relationship between tested class (STLedgerEntry) and unit-test (Invariants_test) in favour of using mock. The change has no functional impact.

Type of Change

• [x] Removed Invariants_test as a friend from STLedgerEntry.
• [x] Declared a mock class that avoids checking LedgerFormats on construction.
• [x] Made a dummy Nickname ledger entry to trigger an invalid ledger entry error.

API Impact

• None