runtime/pprof: try to use real stack in TestTryAdd

TestTryAdd is particularly brittle because it tests some real cases by
constructing fake sample stack frames. If those frames don't correctly
represent what the runtime would generate then they may fail to catch
regressions.

Instead, call runtime.Callers at the bottom of real function calls to
generate real frames as a base for truncation, etc in tests. Several of
these tests still have to fake parts of the frames to test the right
thing, but this is a bit less fragile.

This change is equivalent to the original
0dfb0513 (golang.org/cl/227484), except
that the test skips if the test functions aren't inline (e.g., noopt
builders).

Change-Id: Ie9e32b5660cfe28a924f9cfcddcd887ea2effd66
Reviewed-on: https://go-review.googlesource.com/c/go/+/227922


Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>