Add unit tests for isValidSlpTxn

[jcramer]

This convenience method provides extra protection to the user by detecting and preventing some burn scenarios. Unit tests should be implemented in order to cover all of the scenarios where tokens could be accidentally burned.

The tests should include:

• Does not allow token burns for any non-SLP transactions (implemented)
• Does not allow burns for tokens of the same token ID (implemented)
• Does not allow burns for tokens of other token IDs (not implemented)
• Does not allow excessive burns for NFT1 parent when creating NFT1 child (not implemented)