unix: cap RLIMIT_NOFILE soft limit in TestRlimit on darwin
unix: cap RLIMIT_NOFILE soft limit in TestRlimit on darwin

On some machines, kern.maxfilesperproc is 4096. If Rlimit.Cur is larger
than that, Setrlimit will get an errEINVAL.

Same as CL 246658 did in package syscall.

Updates golang/go#40564

Change-Id: I8c45a23352fa2039772e04205680640e8a0e1840
Reviewed-on: https://go-review.googlesource.com/c/sys/+/250000
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>

golang/sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys