fix(cdk/scrolling): fixed-size-virtual-scroll wrong range and position when items length changes and current scroll is greater than new data length

[albyrock87]

FixedSizeVirtualScroll rendered range is not correct when going from a large list to a smaller list.

I've checked MR #19030 by @crisbeto with the modified test case but it fails anyway.

One of the main problems is in firstVisibleIndex that it's not correct anymore when the list shrinks to a size smaller then the current scroll position, in fact the following code causes newRange.end < newRange.start:

        if (expandEnd > 0) {
          newRange.end = Math.min(dataLength, newRange.end + expandEnd);
          newRange.start = Math.max(0,
              Math.floor(firstVisibleIndex - this._minBufferPx / this._itemSize));
        }

Unit test changes:

• Removed a wrong triggerScroll(viewport) on should handle dynamic item array: user doesn't need to scroll the view to see what changed
• Added some style to test cases to debug easily
• Added a test to check no-regression with the new code

Fixes #19029

[mmalerba]

Looks good, thanks for the fix!

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