selftests/harness: Update named initializer syntax

selftests/harness: Update named initializer syntax

The harness was still using old-style GNU named initializer syntax.
Fix this so Clang will stop warning:

seccomp_bpf.c:2924:1: warning: use of GNU old-style field designator extension
      [-Wgnu-designator]
./../kselftest_harness.h:147:25: note: expanded from macro 'TEST'
                        ^
./../kselftest_harness.h:172:5: note: expanded from macro '__TEST_IMPL'
                  fn: &test_name, termsig: _signal }; \
                  ^

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>