runtime/pprof: TestLabelSystemstack parallelLabelHog.func1 must be labeled

runtime/pprof: TestLabelSystemstack parallelLabelHog.func1 must be labeled

The closure in parallelLabelHog should be labeled in a addition to
parallelLabelHog itself. Generally samples on that goroutine land on
labelHog, but there is a small portion of the closure outside of
labelHog.

Fixes #50740.

Change-Id: I363b6d8eec2e6920c215686e2039fce6d5b29a98
Reviewed-on: https://go-review.googlesource.com/c/go/+/380055
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Michael Pratt <mpratt@google.com>