Rename GSUtil subtests to match the style of other name components.

Also remove "slicing" which is gsutil-specific jargon for multithreadedness.

Before -> After:

- `RootFS/slicing=false` -> `RootFS/SingleThreaded`
- `EmptyDir/slicing=true` -> `EmptyDir/Multithreaded`

PiperOrigin-RevId: 718428094

test/kubernetes/benchmarks/gsutil.go

@@ -109,13 +109,13 @@ func RunGSUtil(ctx context.Context, t *testing.T, k8sCtx k8sctx.KubernetesContex
 				option string
 			}{
 				{
-					name:   "slicing=false",
+					name:   "SingleThreaded",
 					option: `-o "GSUtil:sliced_object_download_threshold=0"`,
 				},
 				{
-					// Slicing is enabled by default, so we
-					// don't set any extra options.
-					name: "slicing=true",
+					// Multithreaded downloading is enabled by default, so we don't set
+					// any extra options.
+					name: "MultiThreaded",
 				},
 			} {
 				t.Run(slicing.name, func(t *testing.T) {