[tests] Add distro specific helper decorators

Adds helper decorators to define specific test methods for use on
specific distributions only. Currently two decorators are available:

    @redhat_only	Only run on fedora, centos, or rhel
    @ubuntu_only	Only run on ubuntu or debian

Note that these decorators are only intended for individual `test_*`
methods, and will not function to define distro-specific test classes.

These should make it easier to write plugin test cases where packaging
differences between distributions otherwise makes plugin tests either
impossible needlessly complex to write generically.

Related: #2431

Signed-off-by: Jake Hunsaker <jhunsake@redhat.com>

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