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ARROW-6832: [R] Implement Codec::IsAvailable

Adds `codec_is_available()` and a test skip helper that uses it. Also fills in some missing test coverage nearby.

Closes #5615 from nealrichardson/r-codec-avail and squashes the following commits:

121b39a40 <Neal Richardson> Oops
98f8260a5 <Neal Richardson> Skip more tests without snappy
792cbed7e <Neal Richardson> Add codec_is_available and fill some missing test coverage nearby

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>