Migrate tests to use python scripts instead of rust test helper

[soenkehahn]

This PR tries to get rid of the test_executables_helper in favor of specifying test programs in python. This has some advantages:

• It allows us to specify the test programs in (potentially multi-line) strings within the tests. Currently, some tests are pretty ugly, since the test program is inside a match statement in src/test_executables/helper.rs.
• It's faster. Executing small python snippets is faster than compiling small rust snippets and executing them. Surprisingly, it's also faster than executing rust snippets with rust-script, even when the compilation artifacts are already cached.
• Hopefully, it allows us to get rid of the test_executables_helper and the test_executables feature in Cargo.toml. I think these details ideally shouldn't be in that file.
• Hopefully it allows us to get rid of the bitflags dependency. Not sure though.

There's also some disadvantages:

• You need python3 to execute the tests. Fortunately, that is installed on lots of systems. For example to make this PR run on CI, I didn't have to install anything there.
• You need to know python to write tests. I think this is pretty minor though, since python is fairly easy to learn and google things for, and also you only have to write very small test snippets.

[casey]

If you use single quotes you can avoid raw strings:

Suggested change

	r#"
	import sys
	print("foo", file=sys.stderr)
	"#,
	"
	import sys
	print('foo', file=sys.stderr)
	",

[casey]

Having the code be out-of-line doesn't bother me so much, so I think I still prefer keeping it in Rust vs adding Python. That being said, I don't feel super strongly about it. I can definitely see the readability benefit of having the code be in-line. Overall the PR looks good.