test: expand run.go's errorcheck, make clear which bugs run

Today, if run.go doesn't understand a test header line it just ignores
the test, making it too easy to write or edit tests that are not actually
being run.

- expand errorcheck to accept flags, so that bounds.go and escape*.go can run.
- create a whitelist of skippable tests in run.go; skipping others is an error.
- mark all skipped tests at top of file.

Update #4139.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6549054

test/fixedbugs/bug302.go

@@ -1,5 +1,8 @@
 // $G $D/bug302.dir/p.go && pack grc pp.a p.$A && $G $D/bug302.dir/main.go
 
+// NOTE: This test is not run by 'run.go' and so not run by all.bash.
+// To run this test you must use the ./run shell script.
+
 // Copyright 2010 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.