Add `assertNotThrows`

**Is your feature request related to a problem? Please describe.**

We have `assertThrows` to check if a function throws. When doing such testing I sometimes like to test for the inverse that it doesn't throw, but don't particular care about the return value.

**Describe the solution you'd like**

`assertNotThrows` or something like Node's `assert.doesNotThrow`

**Describe alternatives you've considered**

I guess the workaround is to assert the return value for now.