MFC r315320:

Start adding basic tests for cam(3)

This change contains several negative and positive tests for:
- cam_open_device
- cam_close_device
- cam_getccb
- cam_freeccb

This also contains a test for the failure case noted in bug 217649,
i.e., O_RDWR must be specified because pass(4) requires it.

This test unfortunately cannot assume that cam-capable devices are
present, so the user must explicitly provide a device via
`test_suites.FreeBSD.cam_test_device`. In the future, a test kernel
module might be shipped, or ctl(4) might be used, as a test device
when testing out libcam, which will allow the tests to do away with
having to specify an explicit test device.