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/2031Reviewed-by: Minux Ma <minux@golang.org>

test/fixedbugs/bug248.go

-// +build !nacl,!windows
+// +build !nacl,!plan9,!windows
 // run
 
 // Copyright 2009 The Go Authors. All rights reserved.

test/fixedbugs/bug345.go

-// +build !nacl,!windows
+// +build !nacl,!plan9,!windows
 // run
 
 // Copyright 2011 The Go Authors.  All rights reserved.
@@ -19,7 +19,7 @@ func main() {
 	a, err := build.ArchChar(build.Default.GOARCH)
 	check(err)
 
-	// TODO: If we get rid of errchk, re-enable this test on Windows.
+	// TODO: If we get rid of errchk, re-enable this test on Plan 9 and Windows.
 	errchk, err := filepath.Abs("errchk")
 	check(err)