test: improve gnovm/pkg/gnolang.TestPrecompile

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 gnolang#1636