test: [a-c]: add introductory comments to tests
Very few of the compiler regression tests include a comment
saying waht they do. Many are obvious, some are anything but.
I've started with a-c in the top directory. More will follow once
we agree on the approach, correctness, and thoroughness here.
zerodivide.go sneaked in too.

R=rsc, r
CC=golang-dev
https://golang.org/cl/5656100
diff --git a/test/const1.go b/test/const1.go
index 53c1940..b24d274 100644
--- a/test/const1.go
+++ b/test/const1.go
@@ -4,6 +4,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Verify overflow is detected when using numeric constants.
+// Does not compile.
+
 package main
 
 type I interface{}