test(gnovm/pkg/gnolang): improve TestPrecompile (#1659)

This test actually only tested the private `precompileAST()` function,
not the public `Precompile()` function.

The change makes it call the public function and adds some more cases
described in comments like:
- unknow-realm (no error)
- syntax error
- blacklist (was already done)
- multiple files (not possible, `Precompile()` is single file only)

Also used multiple-line string for file and expected content, for better
readability.

Relates to #1636

The test can be run with 
```
go test ./gnovm/pkg/gnolang/ -v -run Precompile
```

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pull request. -->

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

- [x] 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
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [x] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>