os: Fix test to work on Solaris.

On Solaris /bin is a symlink to /usr/bin, so running "pwd" in
the directory "/bin" prints out "/usr/bin".

R=rsc, r, bradfitz
CC=golang-dev
https://golang.org/cl/4559043

src/pkg/os/os_test.go

@@ -442,7 +442,8 @@ func exec(t *testing.T, dir, cmd string, args []string, expect string) {
 	var b bytes.Buffer
 	io.Copy(&b, r)
 	output := b.String()
-	if output != expect {
+	// Accept /usr prefix because Solaris /bin is symlinked to /usr/bin.
+	if output != expect && output != "/usr"+expect {
 		t.Errorf("exec %q returned %q wanted %q",
 			strings.Join(append([]string{cmd}, args...), " "), output, expect)
 	}