netutil: skip TestLimitListener on plan9
netutil: skip TestLimitListener on plan9

This may be a platform bug, but nobody has investigated it since the
issue was filed in 2017. Skip it to reduce noise from flakes.

For golang/go#22926

Change-Id: I8f310a539992c4552abb50fea3c7adc2149818ef
Reviewed-on: https://go-review.googlesource.com/c/net/+/369054
Trust: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/net