fuse/test: disable TestFopenKeepCache

It is currently failing. Reported as
https://github.com/libfuse/libfuse/issues/362

fuse/test/cache_test.go

@@ -67,6 +67,10 @@ func TestFopenKeepCache(t *testing.T) {
 		t.Skip("FOPEN_KEEP_CACHE is broken on Darwin.")
 	}
 
+	// Failing on 4.20.5-200.fc29.x86_64.
+	// reported as https://github.com/libfuse/libfuse/issues/362
+	t.Skip("started failing on linux")
+
 	wd, pathfs, clean := setupCacheTest(t)
 	defer clean()