kunit: Place 'test.log' under the 'build_dir'

'kunit' writes the 'test.log' under the kernel source directory even
though a 'build_dir' option is given.  As users who use the option might
expect the outputs to be placed under the specified directory, this
commit modifies the logic to write the log file under the 'build_dir'.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>