When a subproject is disabled on the initial configuration we should not
add it into self.coredata.initialized_subprojects because that will
prevent calling self.coredata.init_builtins() on a reconfigure if the
subproject gets enabled.
Fixes: #10225.
Adds a new debug() function that can be used in the meson.build to
log messages to the meson-log.txt that will not be printed to stdout
when configuring the project.
This adds a new category of tests that does not need to run on all
platforms during CI. For now only run them on Linux runners because they
are not the bottleneck.
Jussi Pakkanen