Adjust timestamps in TestABIChecking to make sure that the library and
executable are rebuilt when expected.

Change-Id: I3288c254ba8201b5b4255347b0cb056fa0908657
Reviewed-on: https://go-review.googlesource.com/11128Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Yves Junqueira <yves.junqueira@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>