test: update test for issue 15548

Accidentally checked in the version of file c.go that doesn't
exhibit the bug - hence the test was not testing the bug fix.
Double-checked that this version exposes the bug w/o the fix.

Change-Id: Ie4dc455229d1ac802a80164b5d549c2ad4d971f5
Reviewed-on: https://go-review.googlesource.com/22837
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

test/fixedbugs/issue15548.dir/c.go

@@ -5,6 +5,6 @@
 package c
 
 import (
-	_ "./a"
 	_ "./b"
+	_ "./a"
 )