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.

Reviewed by: asomers, ken (earlier diff)
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: D9928