add tests for boost::optional casters

[Skylion007]

Description

• Adds tests for boost::optional casters to test suite as well.

Suggested changelog entry:

* Adds tests for boost::optional when compiling pre-C++17 tests.

[Skylion007]

@ryancahoon-zoox something like this (although I am running into a bit of trouble backporting nullopt at the moment).

[henryiii]

I think this was included in #3376.