[Tests] Enemy Held Item overrides are now applied after generateEnemyModifiers()

[frutescens]

What are the changes the user will see?

None.

Why am I making these changes?

This change is needed to test item-related moves and abilities without having unwanted items interfere with test conditions.

What are the changes from a developer perspective?

overrideModifiers and overrideHeldItems are now applied after generateEnemyModifiers() in encounter-phase.ts. Previously, they were applied in addEnemyPokemon(), but addEnemyPokemon() was run before generateEnemyModifiers() so the items in the override would be given but additional items could also be generated afterwards.

Checklist

• I'm using beta as my base branch
• There is no overlap with another PR?
• The PR is self-contained and cannot be split into smaller PRs?
• Have I provided a clear explanation of the changes?
• Have I tested the changes manually?
• Are all unit tests still passing? (npm run test)

[frutescens]

These flakey tests...