Skip message_get_many schema test until we can repro reliably

While we try to track down the root cause of this random test failure,
this patch temporarily skips that test. We don't know yet whether it
it is a false positive or what.

Change-Id: Iabad78da94cf1e3033f3e03c20d2aa1ea8d8a5e1
Related-Bug: #1273335