Part of the 2D scrolling proposal: flutter.dev/go/2D-Foundation

Companion to #122357

Fixes #122355

For the relocated classes in #122357, this relocates the associated tests that were in scrollable_test.dart. The test files for scrollable are already a bit spread out (see test/scrollable_*), so only the one that were in scrollable_test.dart were moved.
These are the classes moved in the other PR:

• ScrollableDetails
  • Not directly tested, nothing to move. Will add oodles of tests though as part of Expand ScrollableDetails to include more info #122415
• EdgeDraggingAutoScroller
  • These tests live elsewhere - scrollable_selection_test.dart
• ScrollIncrementCalculator
  • tests moved in this PR
• ScrollIncrementType
  • tests moved in this PR
• ScrollIncrementDetails
  • tests moved in this PR
• ScrollIncrementIntent
  • tests moved in this PR
• ScrollAction
  • tests moved in this PR

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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