Rework ffi testcases by new prove test scaffold

Now all ffi testcases moved to test/ffi_test.t,
directory test/ffi_test/ contains cparser_test and
dummy kernel module used for test/ffi_test.t.

Compilation of ffi dummy kernel module and cparser_test.c
is triggered by test/ffi_test.t.

If no ffi compiled in, then test/ffi_test.t will be skipped.

Using skip feature for ffi test is suggested by Yichun (agentzh).

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>