Loosen assertEquals type constraint to allow subtyping

[olafurpg]

Context: https://gitter.im/scalameta/munit?at=5e36e5cadc52c34ace3fd552

Currently, the following code does not typecheck

assertEquals(List(1).headOption, Some(1))
// Cannot prove that Option[Int] =:= Some[Int]

We might want to allow the right-hand side (expected value) to be a subtype of the right hand side

assertEquals(List(1).headOption, Some(1)) // OK
assertEquals(Some(1), List(1).headOption) // ERROR: Cannot prove that Option[Int] <:< Some[Int]

The reasoning being that the expected value is often a hardcoded expression like "Some(1)" while the obtained value comes from a generic method implementation.

[olafurpg]

I'm gonna close this as wontfix. If there is demand, I'm open to add a new assertAnyEquals() helpers that takes Any arguments so that it's possible to for example compare Left[Int, Nothing] with Either[Int, Nothing] but it's not a method that I have found the need for so far.

[olafurpg]

Implemented this change in #70. @gabro also found a nice workaround when you really want to compare different types: assertEquals[Any, Any](a, b)