Organise and add more tests

[0xjac]

All tests are located in a single file (test/ReferenceToken-test.js) which is growing. It is becoming hard to read each test case.

The tests should be split in different files, for example:

• Base Tests (Read only functions: name, symbol, ...)
• Send/Receive Tests
• Operator Tests
• ERC-20 Compatibility Tests

[0xjac]

Partially done in #40 but more tests are needed, which is why this remain open