runtime/pprof: assert that labels never appear on unexpected samples

This makes TestLabelSystemstack much more strict, enabling it to detect
any misplacement of labels.

Unfortunately, there are several edge cases where we may not have an
obviously correct stack trace, so we generally except the runtime
package, with the exception of background goroutines that we know should
not be labeled.

For #50007
For #50032

Change-Id: I8dce7e7da04f278ce297422227901efe52782ca0
Reviewed-on: https://go-review.googlesource.com/c/go/+/369984


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>