Testing: in addition to `assert_eq()`, also provide `expect_eq()`

Some C++ test frameworks have the distinction between [assert and expect](https://google.github.io/googletest/reference/assertions.html): ASSERT aborts a test at the first failure while EXPECT keeps going and fails the test if any of the EXPECT-calls failed.

It would be nice to have the EXPECT functionality available as well in DSLX `#[test]` functions.

This would require to create some side-effects to keep track of any failure, so might need some consideration under the hood for some implementation.