bug: Mirror node parsing and mocha context binding

[ivaylogarnev-limechain]

1. Replace all arrow functions in Mocha tests with function declarations to ensure proper access to Mocha's this context, enabling features like this.skip() and this.timeout(). This fix addresses failing tests caused by the lack of this binding in arrow functions.
2. Introduce BigInt parser for handling Mirror Node data response to accurately process large integer values. This resolves issues where the default JSON parser cannot handle numbers exceeding JavaScript's maximum safe integer limit making some tests to fail.