Add missing structure padding to test

WGSL specifies that the size of a host-shareable structure is a
multiple of the largest alignment of any member in that
structure. While SPIR-V cross doesn't generate this trailing padding,
Tint does, and so we need to pad the host structure to match to avoid
validation errors.

Bug: dawn:571
Change-Id: Id5de89125a74c0469ae69d6e86b4c570bbdb31ab
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/56120
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/tests/end2end/GpuMemorySynchronizationTests.cpp

@@ -668,6 +668,7 @@ TEST_P(MultipleWriteThenMultipleReadTests, OneBuffer) {
         float color0;
         char padding2[256 - sizeof(float)];
         float color1;
+        char padding3[256 - sizeof(float)];
     };
     wgpu::Buffer buffer = CreateZeroedBuffer(
         sizeof(Data), wgpu::BufferUsage::Vertex | wgpu::BufferUsage::Index |