[SPARK-40467][SS] Split FlatMapGroupsWithState down to multiple test …

…suites

### What changes were proposed in this pull request?

This PR proposes to split the FlatMapGroupsWithStateSuite into three pieces:

1. GroupStateSuite <- test the functionality with (Test)GroupState implementation
2. FlatMapGroupsWithStateWithInitialStateSuite <- test E2E cases which are specific to initial state
3. FlatMapGroupsWithStateSuite <- all other cases (E2E cases which don't leverage initial state)

The change is pure extraction - it's cut and paste and no additional code change has been introduced.

### Why are the changes needed?

This would help to maintain the test suite FlatMapGroupsWithStateSuite as it's quite huge.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Refactored test suites.

Closes apache#37907 from HeartSaVioR/SPARK-40467.

Authored-by: Jungtaek Lim <[email protected]>
Signed-off-by: Jungtaek Lim <[email protected]>