runtime: skip TestTimePprof on illumos

runtime: skip TestTimePprof on illumos

On illumos nanotime calls libc, like on the other systems for which
TestTimePprof is skipped.

For #43118

Change-Id: I370d3f098a261185920cb1e3e3402d16200e301a
Reviewed-on: https://go-review.googlesource.com/c/go/+/366737
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>