If this option is enabled, the test step is created with the
EXCLUDE_FROM_MAIN option, and if TEST_BEFORE_INSTALL is enabled, the
install step does not depend on the test step.

This means that the test step is never executed, unless the test target
is explicitly created by calling ExternalProject_Add_StepTarget, or by
passing it with the STEP_TARGETS option, or with the EP_STEP_TARGETS
directory property.