tests: shell: Improve concurrent noflush restore test a bit

The described issue happens only if chain FOO does not exist at program
start so flush the ruleset after each iteration to make sure this is the
case. Sadly the bug is still not 100% reproducible on my testing VM.

While being at it, add a paragraph describing what exact situation the
test is trying to provoke.

Fixes: dac904bdcd9a1 ("nft: Fix for concurrent noflush restore calls")
Signed-off-by: Phil Sutter <phil@nwl.cc>

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