Add unbind tests

[DenisCarriere]

What's changed

• [x] added int64_t to_number(str) method to parse number from memo
• [x] added const to methods
• [x] renamed _self to get_self() (ref: eosio.system)
• [x] ~move [eosio::action] on top of method (ref: eosio.system)~

[[eosio::action]]
void drops::enable(const bool enabled)
{
   require_auth(get_self());

• [x] rename modify_drop_binding => modify_ram_payer

• [x] update method params of check_drop_ownership & check_drop_bound

  void modify_ram_payer( const uint64_t drop_id, const name ram_payer );
  void check_drop_ownership( const uint64_t drop_id, const name owner );
  void check_drop_bound( const uint64_t drop_id, const bool bound );

• [x] rename ram.hpp methods to snake case (add "_")

  • ramcost to => ram_cost
  • ramcostwithfee to => ram_cost_with_fee
  • ramproceedstminusfee to => ram_proceeds_minus_fee

• [x] define standard ERROR_* messages

  • ERROR_INVALID_MEMO => Invalid transfer memo. (ex: "<amount>,<data>")
  • ERROR_DROP_NOT_FOUND => Drop not found.
  • ERROR_SYSTEM_DISABLED => Drops system is disabled.

• [x] add comments for actions @user & @admin

• [x] modify .find() using .get()

auto & drop = drops.get(drop_id, ERROR_DROP_NOT_FOUND.c_str());

New Tests

• [x] add unbind tests
  • Error: not found
  • Error: does not belong to account
  • Error: is not unbound
• [x] add bind tests
  • Error: not found
  • Error: does not belong to account
  • Error: is not bound
• [x] add cancelunbind tests
  • Error: No unbind request
• [x] add on_transfer error tests
  • Error - empty memo
  • Error - invalid number
  • Error - number underflow
  • Error - at least 32 characters