unittests: add CUnit based unittest framework

unittests: add CUnit based unittest framework

CUnit is C version of *unit framework to help write test cases.
https://sourceforge.net/projects/cunit/

unittests/* are compiled only if CUnit exists, and detected on
build time in ./configure like any other build time detection,
by running a simple CUnit initialization code.

Some OS/distros have binary package for CUnit. In case of Fedora
and FreeBSD, they both install shared library (libcunit.so) and
CUnit headers required to compile fio's unittests.
 Fedora:
  # dnf install CUnit
 FreeBSD:
  # pkg install cunit

To build and install CUnit from upstream source, do below.
 # ./bootstrap && make && make install
Note that make install seems to install binaries and headers under
~/CUnitHome/ by default.

After applying actual test cases in the next few commits, running
./unittests/unittest will print results to stdout. These are
examples of test cases, and one can add more tests.

-- Example of unittest results
 # ./unittests/unittest

      CUnit - A unit testing framework for C - Version 2.1-3
      http://cunit.sourceforge.net/

 Suite: lib/memalign.c
   Test: memalign/1 ...passed
 Suite: lib/strntol.c
   Test: strntol/1 ...passed
   Test: strntol/2 ...FAILED
     1. unittests/lib/strntol.c:24  - CU_ASSERT_EQUAL(*endp,'\0')
   Test: strntol/3 ...passed
 Suite: oslib/strlcat.c
   Test: strlcat/1 ...passed
   Test: strlcat/2 ...FAILED
     1. unittests/oslib/strlcat.c:28  - CU_ASSERT_EQUAL(strcmp(dst, ""),0)
 Suite: oslib/strndup.c
   Test: strndup/1 ...passed
   Test: strndup/2 ...passed
   Test: strndup/3 ...passed

 Run Summary:    Type  Total    Ran Passed Failed Inactive
               suites      4      4    n/a      0        0
                tests      9      9      7      2        0
              asserts     18     18     16      2      n/a

 Elapsed time =    0.000 seconds

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>