Tweak buffer_texture_copies:sample_count (#4423)

Previously, the copy size did not span the entire texture, which
violates both the requirement in "validating texture buffer copy"
that the texture not be multisampled, and the requirement in
"validating GPUTexelCopyTextureInfo" that a copy to a multisampled
texture span a complete surface.

This change makes the test more specific to the added requirement for
texture-buffer copies. The other requirement for texture copies is
checked by `validation,encoding,cmds,copyTextureToTexture:multisampled_copy_restrictions`.

Author: andyleiserson

src/webgpu/api/validation/image_copy/buffer_texture_copies.spec.ts

@@ -325,21 +325,21 @@ g.test('sample_count')
     if (sampleCount > 1) {
       usage |= GPUTextureUsage.RENDER_ATTACHMENT;
     }
+
+    const textureSize = { width: 16, height: 1, depthOrArrayLayers: 1 };
     const texture = t.createTextureTracked({
-      size: { width: 16, height: 16 },
+      size: textureSize,
       sampleCount,
       format: 'bgra8unorm',
       usage,
     });
 
-    const uploadBufferSize = 32;
+    const uploadBufferSize = 64;
     const buffer = t.createBufferTracked({
       size: uploadBufferSize,
       usage: GPUBufferUsage.COPY_SRC | GPUBufferUsage.COPY_DST,
     });
 
-    const textureSize = { width: 1, height: 1, depthOrArrayLayers: 1 };
-
     const isSuccess = sampleCount === 1;
 
     if (copyType === 'CopyB2T') {