Move GnoVM testing logic within pkg/gnolang; separate from gno test logic

[thehowl]

#2367 covers more line than the coverage reports marks; this is because the test in debugger_test.go, running the filetests, does not work recursively and as such skips the tests added by the PR.

This brings up a larger issue, which is that we should move all our current testing logic from gnovm/tests into gnovm/pkg/gnolang for what concerns GnoVM testing (we can keep the filetests in tests/files; I actually think we should move txtar tests to tests, so that tests becomes the "integration testing" directory).

Also, we should separate the specific logic to run gno tests from that of running GnoVM tests. See-also #1346 -- we should in general remove filetests as a testing method for users, full stop.