os: adjust test: /bin is a symlink, not a directory, on Solaris.

R=rsc
CC=golang-dev
https://golang.org/cl/4239066

src/pkg/os/os_test.go

@@ -614,7 +614,7 @@ func TestChdirAndGetwd(t *testing.T) {
 	}
 	// These are chosen carefully not to be symlinks on a Mac
 	// (unlike, say, /var, /etc, and /tmp).
-	dirs := []string{"/bin", "/", "/usr/bin"}
+	dirs := []string{"/", "/usr/bin"}
 	for mode := 0; mode < 2; mode++ {
 		for _, d := range dirs {
 			if mode == 0 {