When working on some particular feature, it's nice to be able to disable
all the tests except for the one I'm working on; this is currently
possible by editing tests/CMakeLists.txt, and commenting out the tests
you don't want.
This commit goes a step further by letting you give a list of tests you
do want when invoking cmake, e.g.:
cmake -DPYBIND11_TEST_OVERRIDE="test_issues.cpp;test_pickling.cpp" ..
changes the build to build just those two tests (and changes the `pytest`
target to invoke just the two associated tests).
This persists in the build directory until you disable it again by
running cmake with `-DPYBIND11_TEST_OVERRIDE=`. It also adds a message
after the pytest output to remind you that it is in effect:
Note: not all tests run: -DPYBIND11_TEST_OVERRIDE is in effect