Give the unit tests a big overhaul

Give the unit tests a big overhaul

The unit tests contain a lot of duplicated code and here's an attempt
to reduce code duplication.

This commit does several things:

1. Remove #ifdef HAVE_CMOCKA - we already solve this with automake
   conditionals.

2. Create a set of ISC_TEST_* and ISC_*_TEST_ macros to wrap the test
   implementations, test lists, and the main test routine, so we don't
   have to repeat this all over again.  The macros were modeled after
   libuv test suite but adapted to cmocka as the test driver.

   A simple example of a unit test would be:

    ISC_RUN_TEST_IMPL(test1) { assert_true(true); }

    ISC_TEST_LIST_START
    ISC_TEST_ENTRY(test1)
    ISC_TEST_LIST_END

    ISC_TEST_MAIN (Discussion: Should this be ISC_TEST_RUN ?)

   For more complicated examples including group setup and teardown
   functions, and per-test setup and teardown functions.

3. The macros prefix the test functions and cmocka entries, so the name
   of the test can now match the tested function name, and we don't have
   to append `_test` because `run_test_` is automatically prepended to
   the main test function, and `setup_test_` and `teardown_test_` is
   prepended to setup and teardown function.

4. Update all the unit tests to use the new syntax and fix a few bits
   here and there.

5. In the future, we can separate the test declarations and test
   implementations which are going to greatly help with uncluttering the
   bigger unit tests like doh_test and netmgr_test, because the test
   implementations are not declared static (see `ISC_RUN_TEST_DECLARE`
   and `ISC_RUN_TEST_IMPL` for more details.

NOTE: This heavily relies on preprocessor macros, but the result greatly
outweighs all the negatives of using the macros.  There's less
duplicated code, the tests are more uniform and the implementation can
be more flexible.