chore: add filetest in gnovm/makefile (gnolang#3272)

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<details><summary>Contributors' checklist...</summary>

- [ ] Added new tests, or not needed, or not feasible
- [ ] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
</details>

---------

Co-authored-by: Nathan Toups <[email protected]>

gnovm/Makefile

@@ -97,6 +97,9 @@ _test.stdlibs:
 	go run ./cmd/gno test -v ./stdlibs/...
 
 
+_test.filetest:;
+	go test pkg/gnolang/files_test.go -test.short -run 'TestFiles$$/' $(GOTEST_FLAGS)
+
 ########################################
 # Code gen
 # TODO: move _dev.stringer to go:generate instructions, simplify generate