test/cyclic: try to avoid spurious test failures due to cyclic callbacks

Looking at failed pipelines on
https://dev.azure.com/u-boot/u-boot/_build

, it is easy to find lots of
examples that have failed spuriously for the same reason: The "cyclic
function ... took too long" message being printed in the middle of
some console output, causing the 'expect' machinery to fail to find
the expected output.

This mostly happens on QEMU and/or sandbox, i.e. cases where U-Boot
isn't really the only thing running on the CPU, so random host
scheduling can cause several millisecond delays, and if that just
happens to interrupt the U-boot code while one of those cyclic
callbacks are running, that message will be printed.

This is one attempt at trying to avoid those spurious and annoying
failures: Add a "backdoor" that allows bumping the threshold, and in
the test framework, make sure that every time we boot we set that
threshold to a high enough value that should eliminate 99% of those
warnings.

I chose an env-var-with-callback instead of, say, teaching the
'cyclic' command something new. The latter is not always enabled, so
it would be more complicated to hook up; setting an env var that is
not used for anything is, to quote Douglas Adams, mostly harmless. I
made it "hidden" so as not to mess up any existing "print the whole
evnironment test", and also to sort-of emphasize that this isn't
really meant to be an official interface.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>