test: add tests for escape analysis of interface conversions

The false positives (var incorrectly escapes) are marked with BAD.

Change-Id: If64fabb6ea96de44a1177d9ab12e2ccc579fe0c4
Reviewed-on: https://go-review.googlesource.com/5294
Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/test/escape5.go b/test/escape5.go
index a33daee..1d411b3 100644
--- a/test/escape5.go
+++ b/test/escape5.go
@@ -134,7 +134,8 @@
 		return
 	}
 
-	global = p // should make p leak always
+	// should make p leak always
+	global = p // ERROR "p escapes to heap"
 	return T2{p}
 }