Decrease the execution time of the longest E2E tests

[ilonatommy]

Fixes #64075

• Two E2E virtualization tests take much more time than other top long-running tests.

Root cause:
The test component used 5000 items × 30px = 150,000px of scroll height, requiring ~250 PageDown iterations to scroll through.

Justification:
There is no justification for 5k items, the same check, reflecting the conditions from the original issue #63651 can be done with 500 elements.

Fix:
Reduced to 500 items × 20px = 10,000px (~17 iterations) while preserving the test's purpose: validating that OverscanCount > MaxItemCount triggers elevation of effective max.

Safety measures:
We could go down even more. The fixing PR mentioned that the core of the problem was with OverscanCount > MaxItemCount. We could use e.g. 100 items with OverscanCount=40 and MaxItemCount = 20 that would reduce the test time ~8x. But only if @javiercn does not have any arguments against that breaking the goal of the test.

What's preserved:

• OverscanCount="200" and MaxItemCount="100" (original relationship)
• ItemSize="20" (matches the Aspire issue that triggered the original fix)
• Assertion >= 200 items rendered initially
• Full scroll-through validation of contiguity

[Copilot]

Pull request overview

This PR optimizes the execution time of E2E virtualization tests by reducing the data size in the test component while preserving the test's core validation logic.

• Reduces item count from 5000 to 500 items (10x reduction)
• Changes ItemSize from 30px to 20px to match the original Aspire issue context
• Maintains the critical relationship where OverscanCount (200) > MaxItemCount (100)