syscall: unskip TestDirentRepeat on freebsd

syscall: unskip TestDirentRepeat on freebsd

TestDirentRepeat fails on FreeBSD 11, but seems to pass on newer
versions. Go 1.18 is the last release to support FreeBSD 11 per
https://golang.org/doc/go1.18#freebsd and there are no FreeBSD 11
builders anymore. Thus unskip TestDirentRepeat to verify the issue is
indeed fixed on FreeBSD 12 and later.

For #31416

Change-Id: I189ef06719ff830ffe2e402c74a75874c9e5b97b
Reviewed-on: https://go-review.googlesource.com/c/go/+/393618
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>