runtime/pprof: do not require a GOROOT/src prefix in tests

runtime/pprof: do not require a GOROOT/src prefix in tests

When paths are trimmed, the reported file locations begin with the
package import path (not GOROOT/src).

Updates #51461

Change-Id: Idbd408a02e8d03329d10e30b0b08263e69e66285
Reviewed-on: https://go-review.googlesource.com/c/go/+/391812
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>