The TEST_INCLUDE_FILE directory property got deprecated in CMake 3.10
and TEST_INCLUDE_FILES should be used instead. This extends descriptions
a bit to better understand this property.
Add new directory property TEST_INCLUDE_FILES. This supersedes
TEST_INCLUDE_FILE, though the latter is of course retained for
compatibility.
Basically, this is a list rather than a single file. This allows the
feature to be used by generic utilities without conflicting with local
use.