Fix turbulence seed for all tests with ink sparkles (#138757)

Fixes flutter/flutter#138753

This makes sure that InkSparkles are always fixed for testing rather than random.

packages/flutter/lib/src/material/ink_sparkle.dart

@@ -201,6 +201,13 @@ class InkSparkle extends InteractiveInkFeature {
 
     // Creates an element of randomness so that ink emanating from the same
     // pixel have slightly different rings and sparkles.
+    assert((){
+      // In tests, randomness can cause flakes. So if a seed has not
+      // already been specified (i.e. for the purpose of the test), set it to
+      // the constant turbulence seed.
+      turbulenceSeed ??= _InkSparkleFactory.constantSeed;
+      return true;
+    }());
     _turbulenceSeed = turbulenceSeed ?? math.Random().nextDouble() * 1000.0;
   }
 
@@ -429,7 +436,9 @@ class InkSparkle extends InteractiveInkFeature {
 class _InkSparkleFactory extends InteractiveInkFeatureFactory {
   const _InkSparkleFactory() : turbulenceSeed = null;
 
-  const _InkSparkleFactory.constantTurbulenceSeed() : turbulenceSeed = 1337.0;
+  const _InkSparkleFactory.constantTurbulenceSeed() : turbulenceSeed = _InkSparkleFactory.constantSeed;
+
+  static const double constantSeed = 1337.0;
 
   static void initializeShader() {
     if (!_initCalled) {

packages/flutter/test/material/ink_sparkle_test.dart

@@ -179,7 +179,7 @@ Future<void> _runM3Test(WidgetTester tester, String positionName, double distanc
           key: repaintKey,
           child: ElevatedButton(
             key: buttonKey,
-            style: ElevatedButton.styleFrom(splashFactory: InkSparkle.constantTurbulenceSeedSplashFactory),
+            style: ElevatedButton.styleFrom(),
             child: const Text('Sparkle!'),
             onPressed: () { },
           ),