tests: rewrite pipe syscall decoding check from match_grep to match_diff

tests: rewrite pipe syscall decoding check from match_grep to match_diff

* configure.ac (AC_CHECK_FUNCS): Remove pipe2.
* tests/pipe.c: Include <asm/unistd.h>, skip the test if [!__NR_pipe]
instead of [!HAVE_PIPE2].
(main): Do not call pipe2.
* tests/pipe.test: Skip the test if libc pipe wrapper does not use
pipe syscall, rewrite from match_grep to match_diff.
* tests/pipe.expected: Update expected output.