REGTESTS: server: add test for 'set server name' CLI command

REGTESTS: server: add test for 'set server name' CLI command

Tests cover:
  - error cases: missing name, not-in-maintenance, invalid chars
    (rejected by invalid_char()), duplicate name in the same backend,
    name-referenced server (use-server target, track target)
  - same-name rename as a no-op success
  - successful rename with verification via 'show servers state'
  - old name no longer resolves after rename
  - round-trip rename back to original name
  - traffic still works after rename round-trip

The use-server and tracked-server cases exercise the SRV_F_NAME_REFD
gating added in the preceding patch. Servers pinned only via resolvers
(SRV_F_NON_PURGEABLE without SRV_F_NAME_REFD) remain renamable; that
positive case is not exercised here as it would require a real DNS
resolver in the test environment.