[PATCH] test: session keys are known broken without session key support

[Daniel Kahn Gillmor]

On Tue 2017-12-05 13:40:27 -0500, Daniel Kahn Gillmor wrote:
> If the version of GMime we're building against doesn't support session
> key extraction or re-use, mark the tests that rely on session key
> capabilities as known-broken.
>
> This should resolve test suite failures on ubuntu trusty and debian
> jessie and earlier, which have GMime 2.6.20 -- session key support was
> introduced in GMime 2.6.21.

This should fix the current travis build failure.  If it's accepted,
i'll use the same technique to mark other tests in my session-keys
series as dependent upon session-key capability.

       --dkg