Add expr active item test

[F1r3w477]

Problem

The Skript project has low test coverage. The ExprActiveItem expression had no automated tests.

Solution

This PR adds tests for ExprActiveItem, covering its behavior with single and multiple entities, as well as syntax aliases.

Testing Completed

Added the ExprActiveItem.sk syntax test. The full project test suite was run via .\gradlew skriptTest and passed successfully.

Supporting Information

The positive case (an entity actively using an item) was intentionally not tested, as automating specific AI behavior can be unreliable.

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Completes: none
Related: #6158

[Burbulinis]

What about tests for setting the entity's active item and confirming that they're correct?

[F1r3w477]

What about tests for setting the entity's active item and confirming that they're correct?

From my PR description:

The positive case (an entity actively using an item) was intentionally not tested, as automating specific AI behavior can be unreliable.

[APickledWalrus]

Tests look good, but I don't think they need to be separate in separate "test" cases. One would suffice (I would call it "ExprActiveItem" or "Active Item")

[F1r3w477]

I restructured to have one test that is "Active Item" for the single entities and another that is "Active Item on Multiple Entities" for multiple entities

[sovdeeth]

I don't think this is worth adding. There's no test for the actual behavior the syntax provides, since as you said it's not easily testable.

[F1r3w477]

The test was quick to make. It's here to add if you all want it. If not, that's fine. If everyone decides it shouldn't be added, should it be tracked somewhere that it's not testable?