Optimize eval tests

[mtshiba]

Describe the code issue?

Currently, eval tests heavily call eval function. However, this is not lightweight. To speed up testing, it may be necessary to make eval function reusable or to parallelize tests.

Since cargo executes test functions with the #[test] attribute in parallel, it can be solved naively.

Alternatively, we can rewrite

assert_eq!(eval("print! 1"), successful_output("1\n"));
assert_eq!(eval("print! \"abc\""), successful_output("abc\n"));

to

assert_eq!(eval("print! 1, \"abc\""), successful_output("1 abc\n"));

Additional context

No response

Erg version

0.6.3

Python version

None

OS

None