test: disable bug248 and bug345 tests on Plan 9

These tests were enabled as part of change 1774.
They depend on the errchk tool, which is a Perl
script. However, Perl is not available on Plan 9.

Change-Id: I82707aae16013acc9a3800d39b0084588b852b53
Reviewed-on: https://go-review.googlesource.com/2031
Reviewed-by: Minux Ma <minux@golang.org>
diff --git a/test/fixedbugs/bug248.go b/test/fixedbugs/bug248.go
index 13796ee..3d9a408 100644
--- a/test/fixedbugs/bug248.go
+++ b/test/fixedbugs/bug248.go
@@ -1,4 +1,4 @@
-// +build !nacl,!windows
+// +build !nacl,!plan9,!windows
 // run
 
 // Copyright 2009 The Go Authors. All rights reserved.