cmake: Expose PYBIND11_TEST_OVERRIDE

[EricCousineau-TRI]

Primarily for the ccmake curses GUI

I've found myself using this quite a bit, and would love to have it a bit more accessible.

[rwgk]

Would it make sense to add a comment to your change in CMakeList.txt, to explain what it is good for?
(I'm not a regular cmake users myself, but I imagine a comment targeted at cmake users could be very helpful.)

[EricCousineau-TRI]

@rwgk I'm not sure if it would make sense, as set(... CACHE ...) / option(...) cache vars are pretty standard for CMake configuration.
https://cmake.org/cmake/help/latest/command/set.html#set-cache-entry
https://cmake.org/cmake/help/latest/command/option.html

@jamiesnape If you have time, can I ask your opinion (on the CMake side)?

[rwgk]

Looks good to me.

[jamiesnape]

I do not think the CMake constructs need documenting, but the docstring in the set(... CACHE) could be improved, I think. Override tests is a little vague. Something like "tests from ;-separated list of *.cpp files will be built instead of all tests"?

[EricCousineau-TRI]

Done - thanks!

[wjakob]

LGTM, thanks!