tests: bluetooth: btp_mesh: return error when blob target list is full

cmd_blob_target() always returned 0 even when the target list was
full, making error checks at call sites unreachable.

Return a proper error code when no more targets can be added so
callers can correctly detect and handle the failure.

Signed-off-by: Gaetan Perrot <gaetan.perrot@spacecubics.com>