test: restore fixedbugs/bug398.go to original version

Cannot reproduce original problem. Compiler internals
have changed enough such that this appears to work now.

Restore original test (exported interfaces), but also
keep version of the test using non-exported interfaces.

Fixes #15596.

Change-Id: Idb32da80239963242bd5d1609343c80f19773b0c
Reviewed-on: https://go-review.googlesource.com/43622


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>