For instance musl does not recognize the %L modifier.

The added make target is described in HOWTO/TESTING.md.

Ideally we should ignore all read only filesystems,
but this will have to do for now.

We do this as we don't want disk_sup to report that
a filesystem is almost full if it is read-only.

The message signaling that the spinning processes had stopped was
previously sent too early.

Previously, the `util_values` test case in the test suite
`cpu_sup_SUITE` tested the `cpu_sup:util()` by checking if a process
that spins in a loop cause the CPU utilization measured with
`cpu_sup:util()` to increase. This was unreliable on test machines
that ran other tasks at the same time. This commit tries to make the
test case more reliable by skipping the test case if it is detected
that the system is doing other work that use a lot of CPU time and
starting 100 spinning processes instead of only 1.

This reverts commit 425889ba69b69a9b6fb14bfbef121d51b78e853a.