t/zbd: skip tests when test prerequisites are not met

t/zbd: skip tests when test prerequisites are not met

Some of the test cases in t/zbd/test-zbd-support require test target
devices to have certain features. When these prerequisites are not met,
they skip the actual test and report the test result to be "PASS".
This does not help users to understand the true test outcome.
As the tests expand to cover a wider variety of zoned devices and
layouts, reporting skipped tests becomes more and more beneficial.

Modify test-zbd-support script to report skipped test cases.
Introduce helper functions require_*() to check test target
prerequisites. If they are not met, set the variable SKIP_REASON and
return the constant SKIP_TESTCASE from the test function. In the main
loo, print "SKIP" status and SKIP_REASON if the test case is skipped.
Also, output the total number of skipped cases at the end of the test
script  run.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>