Use string.inspect on Literal dynamic value instead of a `"literal" string

[quentin-bettoum]

In some cases, the gleam test command don't print the right result when the assertion fails:

 code: assert void_html_sample() == "<some_tag>"
 left: "<some_tag/>"
right: literal
 info: Assertion failed.

Is there a specific reason for this literal string?
If not, this change prints the dynamic value instead, which I feel make it easier to see the difference:

 code: assert void_html_sample() == "<some_tag>"
 left: "<some_tag/>"
right: "<some_tag>"
 info: Assertion failed.

It would be even nicer with diff coloring, like in Elixir, but that's a level of complexity I'm not ready to take on for now 🙂

[lpil]

Thank you!