Start testing the interpreter

[osa1]

We should start testing the interpreter.

The tests should be written in a way that is possible to run in the bootstrapped compiler as well.

My current thinking is that we should have everything needed to evaluate output of a program in the same source as the program. The test runner should parse the comments in the file for: exit code, stderr and stdout outputs.

In the interpreter, we can probably use golden-tests or something similar.

In the bootstrapped compiler we can keep using the same library, or port it to Fir.

[osa1]

I've started adding some tests in https://github.com/fir-lang/fir/tree/main/tests, using golden-tests.

We should add more.