diff: refresh viewers in headless tests

DiffViewerBase no longer calls 'rediff' in 'init' if component is not shown.

Fix regression after d12e363559743f0682e92768fe97333bd1ce5ba4

GitOrigin-RevId: 5053460e8fc6c888c010b65a2e3bd979d5ad30b4

platform/diff-impl/tests/testSrc/com/intellij/diff/merge/MergeTestBase.kt

@@ -59,6 +59,7 @@ abstract class MergeTestBase : HeavyDiffTestCase() {
     val viewer = TextMergeTool.INSTANCE.createComponent(context, request) as TextMergeViewer
     try {
       val toolbar = viewer.init()
+      viewer.viewer.rediff()
       UIUtil.dispatchAllInvocationEvents()
 
       val builder = TestBuilder(viewer, toolbar.toolbarActions ?: emptyList())

platform/diff-impl/tests/testSrc/com/intellij/openapi/diff/impl/settings/DiffColorSettingsTest.kt

@@ -27,6 +27,7 @@ class DiffColorSettingsTest : HeavyDiffTestCase() {
     try {
       panel = DiffPreviewPanel()
       val viewer = panel.testGetViewer()
+      viewer.rediff()
 
       assertEquals(viewer.changes.size, 6)
       assertContainsRange(viewer, TextDiffType.MODIFIED)