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.
This argument allows to set default arguments that are written in the
initial cache file, but that are not forced, and therefore allows the
user to change these values later (CMAKE_ARGS and CMAKE_CACHE_ARGS
always overwrite the values).
Also add some documentation to explain the differences between these 3
arguments.
Extend ExternalProjectLocal test to cover CMAKE_CACHE_DEFAULT_ARGS.
Move the subtests that test with local projects into their own test. The reason
is that on slower or crowded machines the test may reach the limit of 25
minutes and therefore fail while it would pass if it was given enough time.
The split is roughly 3:1 with regard to the execution time, with the new
ExternalProjectLocal test being the faster one.
