swift: adding unit tests

[reez]

Description

Adding Unit Tests for Swift

Notes to the reviewers

Unit tests will try to include as much:

• parity with JVM / Android
  • added a few private variables for reusability (similar to JVM/Android)
  • updated testMemoryWalletNewAddress to use testnet (similar to JVM/Android)
  • added testMemoryWalletBalance (similar to JVM/Android)
• parity with bitcoindevkit-scripts Swift

Can run swift test on this file via command line.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR