new cassandane feature: skip tests based on cyrus version being tested

[ellie timoney]

I've just pushed up a new feature in Cassandane for marking tests as
only applying to particular version ranges.  So if you're writing tests
for a new Cyrus feature, you can mark them appropriately, and Cassandane
can still run cleanly when testing versions prior to that feature's
introduction.

There's two new magical subroutine attribute patterns:
        :min_version_x_y_z
        :max_version_x_y_z
(where in both cases y and z are optional).

These only apply to test suites inheriting from
Cassandane::Cyrus::TestCase.  Test suites inheriting from
Cassandane::Unit::TestCase will ignore these attributes entirely -- but
you probably shouldn't inherit from this anyway (unless you're testing
Cassandane itself).

So for example, you might test a feature that's new in master with
something like:

    sub test_my_new_feature
        :min_version_3_0
    {
         # [...]
    }

And you might continue to test some hypothetical feature that's been
discontinued on master but still exists in the stable branch with
something like:

    sub test_my_obsolete_feature
        :max_version_2_5
    {
        # [...]
    }

There's also a new class method on Cassandane::Instance: get_version(). 
It's able to detect versions as far back as 2.5.0.  So if you need to do
some version-based conditionalisation within a test function (or within
infrastructure), you can use something like:

    my ($major, $minor, $revision, $extra) =
    Cassandane::Instance->get_version()
    # [...]

And there's a new infrastructure test suite, Cassandane::Test::Skip,
which tests the implementation of the skip handling.

The end goal here is to no longer need a separate "for-cyrus-2.5" branch
in Cassandane -- I want to be able to use the same test suite for
testing all future releases (I don't plan for it to support 2.4 or
earlier).  There's still some work to go in this respect, in terms of
attributing our existing tests appropriately and other little tweaks to
accommodate 2.5.  But this feature now exists for use in development of
new tests.

I've already converted the Archive, Delete and JMAP test suites to
attribute their version requirements appropriately (so have a look at my
recent commits to these modules for real world examples), and am working
through the rest (low hanging fruit first).

Cheers,

ellie