kunit: Fix '--build_dir' option

kunit: Fix '--build_dir' option

Running kunit with '--build_dir' option gives following error message:

```
$ ./tools/testing/kunit/kunit.py run --build_dir ../linux.out.kunit/
[00:57:24] Building KUnit Kernel ...
[00:57:29] Starting KUnit Kernel ...
Traceback (most recent call last):
  File "./tools/testing/kunit/kunit.py", line 136, in <module>
    main(sys.argv[1:])
  File "./tools/testing/kunit/kunit.py", line 129, in main
    result = run_tests(linux, request)
  File "./tools/testing/kunit/kunit.py", line 68, in run_tests
    test_result = kunit_parser.parse_run_tests(kunit_output)
  File "/home/sjpark/linux/tools/testing/kunit/kunit_parser.py", line
283, in parse_run_tests
    test_result =
parse_test_result(list(isolate_kunit_output(kernel_output)))
  File "/home/sjpark/linux/tools/testing/kunit/kunit_parser.py", line
54, in isolate_kunit_output
    for line in kernel_output:
  File "/home/sjpark/linux/tools/testing/kunit/kunit_kernel.py", line
145, in run_kernel
    process = self._ops.linux_bin(args, timeout, build_dir)
  File "/home/sjpark/linux/tools/testing/kunit/kunit_kernel.py", line
69, in linux_bin
    stderr=subprocess.PIPE)
  File "/usr/lib/python3.5/subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1551, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: './linux'
```

This error occurs because the '--build_dir' option value is not passed
to the 'run_kernel()' function.  Consequently, the function assumes
the kernel image that built for the tests, which is under the
'--build_dir' directory, is in kernel source directory and finally raises
the 'FileNotFoundError'.

This commit fixes the problem by properly passing the '--build_dir'
option value to the 'run_kernel()'.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>