test: improve gnovm/pkg/gnolang.TestPrecompile

This test was actually only testing the private `precompileAST` function
and not the public `Precompile` function.

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

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

Relates to gnolang#1636