Fill in the unit test of implemented bif

[RyanSiu1995]

Some of the unit tests are not implemented yet. Required someone to fill them up.

• [ ] Math functions
• [ ] List functions
• [ ] Chrono functions

Here are the incomplete unit tests (without testing the exception case).

• [ ] erlang functions

[archseer]

Agreed! I think it's important to have some coverage, but in the future, we'll also cover some part of that via tests inside the runtime

[archseer]

I'm also going to set up code coverage reporting