tests: Disable glibc memory checking under glibc <= 2.11.

We noticed that the unit tests sometimes fail on XenServer inside glibc's
memory checker, in the free_check() function.  It turns out that the
glibc memory checker in glibc 2.11 and earlier had an internal race that
caused false positives in multithreaded programs.

This commit avoids the problem by disabling the glibc memory checker in
glibc 2.11 and earlier.

VMware-BZ: #1267127
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>