nfsmount: require --vfs-cache-mode writes or above in tests

These tests fail for --vfs-cache-mode minimal on Linux for the same
reason they don't work properly with --vfs-cache-mode off

cmd/nfsmount/nfsmount_test.go

@@ -28,5 +28,5 @@ func TestMount(t *testing.T) {
 		}
 		sudo = true
 	}
-	vfstest.RunTests(t, false, vfscommon.CacheModeMinimal, false, mount)
+	vfstest.RunTests(t, false, vfscommon.CacheModeWrites, false, mount)
 }