[Impeller] updated gaussian blur tests to use device private textures (#48417)

fixes https://github.com/flutter/flutter/issues/138955

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- [x] I added new tests to check the change I am making or feature I am
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- [x] All existing and new tests are passing.

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impeller/entity/contents/filters/directional_gaussian_blur_filter_contents_unittests.cc

diff --git a/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents_unittests.cc b/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents_unittests.cc
index c91ae49..7493a81 100644
--- a/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents_unittests.cc
+++ b/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents_unittests.cc
@@ -89,6 +89,7 @@
 TEST_P(DirectionalGaussianBlurFilterContentsTest,
        RenderCoverageMatchesGetCoverage) {
   TextureDescriptor desc = {
+      .storage_mode = StorageMode::kDevicePrivate,
       .format = PixelFormat::kB8G8R8A8UNormInt,
       .size = ISize(100, 100),
   };

impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc

diff --git a/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc b/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc
index d8378d3..55ccd33 100644
--- a/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc
+++ b/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc
@@ -70,6 +70,7 @@
 
 TEST_P(GaussianBlurFilterContentsTest, CoverageWithTexture) {
   TextureDescriptor desc = {
+      .storage_mode = StorageMode::kDevicePrivate,
       .format = PixelFormat::kB8G8R8A8UNormInt,
       .size = ISize(100, 100),
   };
@@ -88,6 +89,7 @@
 
 TEST_P(GaussianBlurFilterContentsTest, CoverageWithEffectTransform) {
   TextureDescriptor desc = {
+      .storage_mode = StorageMode::kDevicePrivate,
       .format = PixelFormat::kB8G8R8A8UNormInt,
       .size = ISize(100, 100),
   };
@@ -115,6 +117,7 @@
 
 TEST_P(GaussianBlurFilterContentsTest, RenderCoverageMatchesGetCoverage) {
   TextureDescriptor desc = {
+      .storage_mode = StorageMode::kDevicePrivate,
       .format = PixelFormat::kB8G8R8A8UNormInt,
       .size = ISize(100, 100),
   };
@@ -146,6 +149,7 @@
 TEST_P(GaussianBlurFilterContentsTest,
        RenderCoverageMatchesGetCoverageTranslate) {
   TextureDescriptor desc = {
+      .storage_mode = StorageMode::kDevicePrivate,
       .format = PixelFormat::kB8G8R8A8UNormInt,
       .size = ISize(100, 100),
   };
@@ -159,6 +163,7 @@
   entity.SetTransform(Matrix::MakeTranslation({100, 200, 0}));
   std::optional<Entity> result =
       contents->GetEntity(*renderer, entity, /*coverage_hint=*/{});
+
   EXPECT_TRUE(result.has_value());
   if (result.has_value()) {
     EXPECT_EQ(result.value().GetBlendMode(), BlendMode::kSourceOver);
@@ -178,6 +183,7 @@
 TEST_P(GaussianBlurFilterContentsTest,
        RenderCoverageMatchesGetCoverageRotated) {
   TextureDescriptor desc = {
+      .storage_mode = StorageMode::kDevicePrivate,
       .format = PixelFormat::kB8G8R8A8UNormInt,
       .size = ISize(400, 300),
   };
@@ -210,6 +216,7 @@
 
 TEST_P(GaussianBlurFilterContentsTest, CalculateUVsSimple) {
   TextureDescriptor desc = {
+      .storage_mode = StorageMode::kDevicePrivate,
       .format = PixelFormat::kB8G8R8A8UNormInt,
       .size = ISize(100, 100),
   };