Added utilities to read CMAKE_CFG_INT_DIR from tests.

Use cmake_export_cfg_int_dir (FILENAME) to write out the current
CFG_INT_DIR to FILENAME at build time.

Use cmake_import_cfg_int_dir (FILENAME) to read the current
CFG_INT_DIR during the verify stage.

This is particularly useful when attempting to read files created
inside of external projects in the verify stage.