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>

test/fixedbugs/bug398.go

@@ -5,16 +5,36 @@
 // license that can be found in the LICENSE file.
 
 // Used to crash compiler in interface type equality check.
+// (This test used to have problems - see #15596.)
 
 package p
 
+// exported interfaces
+
+type I1 interface {
+      F() interface{I1}
+}
+
+type I2 interface {
+      F() interface{I2}
+}
+
+var V1 I1
+var V2 I2
+
+func F() bool {
+       return V1 == V2
+}
+
+// non-exported interfaces
+
 type i1 interface {
       F() interface{i1}
 }
 
 type i2 interface {
       F() interface{i2}
-}       
+}
 
 var v1 i1
 var v2 i2
@@ -22,6 +42,3 @@ var v2 i2
 func f() bool {
        return v1 == v2
 }
-
-// TODO(gri) Change test to use exported interfaces.
-// See issue #15596 for details.
\ No newline at end of file