test: expand issue7863 test

This was sitting in my client but I forgot hg add.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/101800045

test/fixedbugs/issue7863.go

@@ -6,12 +6,55 @@
 
 package main
 
-import "time"
+import (
+	"fmt"
+)
+
+type Foo int64
+
+func (f *Foo) F() int64 {
+	return int64(*f)
+}
+
+type Bar int64
+
+func (b Bar) F() int64 {
+	return int64(b)
+}
+
+type Baz int32
+
+func (b Baz) F() int64 {
+	return int64(b)
+}
 
 func main() {
-	now := time.Now()
-	f := now.Unix
-	if now.Unix() != f() {
-		println("BUG: ", now.Unix(), "!=", f())
+	foo := Foo(123)
+	f := foo.F
+	if foo.F() != f() {
+		bug()
+		fmt.Println("foo.F", foo.F(), f())
+	}
+	bar := Bar(123)
+	f = bar.F
+	if bar.F() != f() {
+		bug()
+		fmt.Println("bar.F", bar.F(), f()) // duh!
+	}
+
+	baz := Baz(123)
+	f = baz.F
+	if baz.F() != f() {
+		bug()
+		fmt.Println("baz.F", baz.F(), f())
+	}
+}
+
+var bugged bool
+
+func bug() {
+	if !bugged {
+		bugged = true
+		fmt.Println("BUG")
 	}
 }