testing: use function names to identify helpers

testing: use function names to identify helpers

Previously, helpers were identified by entry PC, but this breaks if the
helper is inlined (as in notHelperCallingHelper). Instead, identify
helpers by function name (with package path). Now TestTBHelper and
TestTBHelperParallel pass with -l=4.

To keep the code unified, this change makes it so that the runner
is also identified by function name instead of entry PC.

Change-Id: I1b1987fc49d114e69d075fab56aeeacd5294982b
Reviewed-on: https://go-review.googlesource.com/41257
Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>