tc-testing: very simple example test cases

As part of documentation, supply some very simple test cases
to illustrate how test cases work.  One test case shows
commands in the setup, command, verify and teardown stages.
Other test cases show how to have a working test case that
does not have commands in the setup, verify and/or teardown
stages.

Specifically, the command lists for setup and teardown can
be empty.  And the verify command must have a command, but
it can be /bin/true.  The regex must have a string, we
recommend a single space, and the count of matches must be
zero if you do not want to use the match feature of verify.
Verify will always look for a return code of success (0)
so we give /bin/true when we do not want to make a check
there.

Also, update the documentation for testcases to be more
specific in the cases of:

    - accepting non-success return codes in setup and
      teardown stages
    - how to write the test when no setup, teardown
      and/or verify are desired.

To run the example test cases:

    $ sudo -E ./tdc.py -f creating-testcases/example.json -l
    1f: (example) simple test to test framework
    2f: (example) simple test, no need for verify
    3f: (example) simple test, no need for setup or teardown (or verify)
    $ sudo -E ./tdc.py -f creating-testcases/example.json
    Test 1f: simple test to test framework
    Test 2f: simple test, no need for verify
    Test 3f: simple test, no need for setup or teardown (or verify)
    All test results:

    1..3
    ok 1 1f simple test to test framework
    ok 2 2f simple test, no need for verify
    ok 3 3f simple test, no need for setup or teardown (or verify)

    $

Signed-off-by: Brenda J. Butler <bjb@mojatatu.com>
Acked-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>