runtime/pprof: increase systemstack calls in TestLabelSystemstack

TestLabelSystemstack needs to collect samples within runtime.systemstack
to complete the test.

The current approach uses fmt.Fprintf, which gets into systemstack
through the allocator and GC, but also does lots of other work. In my
measurements, approximately 2% of samples contain runtime.systemstack.

The new approach uses debug.SetGCPercent, which uses systemstack for
most of its work, including contention on mheap_.lock, which extends
usage even more. In my measurements, approximately 99% of samples
contain runtime.systemstack.

Fixes #50050

Change-Id: I59e5bb756341b716a12e13d2e3fe0adadd7fe956
Reviewed-on: https://go-review.googlesource.com/c/go/+/370375


Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>