[ovsp4rt] Update unit tests for wide VNIs

[ffoulkes]

• Widened a temporary variable from 16 to 32 bits.
• Added a couple of 24-bit VNI test cases.

Extracted from PR https://github.com/ipdk-io/networking-recipe/pull/616 so the test updates can be merged separately.

[!NOTE] These are safe changes. They augment what they unit tests support. They have no effect on encodings or the public API.