cmake: Expose PYBIND11_TEST_OVERRIDE

Primarily for the ccmake curses GUI

Author: EricCousineau-TRI

tests/CMakeLists.txt

@@ -8,6 +8,7 @@
 cmake_minimum_required(VERSION 2.8.12)
 
 option(PYBIND11_WERROR  "Report all warnings as errors"  OFF)
+set(PYBIND11_TEST_OVERRIDE "" CACHE STRING "Override unittests. Please specify using *.cpp files")
 
 if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
     # We're being loaded directly, i.e. not via add_subdirectory, so make this