tests: fix typo in TestFopenKeepCache

./cache_test.go:98: Skip call has possible formatting directive %d
FAIL	github.com/hanwen/go-fuse/fuse/test [build failed]

fuse/test/cache_test.go

@@ -95,7 +95,7 @@ func TestFopenKeepCache(t *testing.T) {
 	}
 
 	if minor := pathfs.Connector().Server().KernelSettings().Minor; minor < 12 {
-		t.Skip("protocol v%d has no notify support.", minor)
+		t.Skipf("protocol v%d has no notify support.", minor)
 	}
 
 	code := pathfs.EntryNotify("", "file.txt")