Remove unnecessary parens in test (#151846)

In order to address [a bug in `unnecessary_parenthesis`](https://github.com/dart-lang/linter/issues/3046), we are reporting a few more cases in `unnecessary_parenthesis`. `a = (b = c)` is one such case. This change is being made in https://dart-review.googlesource.com/c/sdk/+/366891.

Author: srawlins

packages/flutter/test/rendering/editable_test.dart

@@ -1071,11 +1071,11 @@ void main() {
       pumpFrame(phase: EnginePhase.paint);
       expect(currentPainter.paintCount, 1);
 
-      editable.foregroundPainter = (currentPainter = _TestRenderEditablePainter()..repaint = false);
+      editable.foregroundPainter = currentPainter = _TestRenderEditablePainter()..repaint = false;
       pumpFrame(phase: EnginePhase.paint);
       expect(currentPainter.paintCount, 0);
 
-      editable.foregroundPainter = (currentPainter = _TestRenderEditablePainter()..repaint = true);
+      editable.foregroundPainter = currentPainter = _TestRenderEditablePainter()..repaint = true;
       pumpFrame(phase: EnginePhase.paint);
       expect(currentPainter.paintCount, 1);
     });
@@ -1090,11 +1090,11 @@ void main() {
       pumpFrame(phase: EnginePhase.paint);
       expect(currentPainter.paintCount, 1);
 
-      editable.painter = (currentPainter = _TestRenderEditablePainter()..repaint = false);
+      editable.painter = currentPainter = _TestRenderEditablePainter()..repaint = false;
       pumpFrame(phase: EnginePhase.paint);
       expect(currentPainter.paintCount, 0);
 
-      editable.painter = (currentPainter = _TestRenderEditablePainter()..repaint = true);
+      editable.painter = currentPainter = _TestRenderEditablePainter()..repaint = true;
       pumpFrame(phase: EnginePhase.paint);
       expect(currentPainter.paintCount, 1);
     });