tests: Exclude tests for Windows build.

* tests/Makefile.am [HAVE_W32_SYSTEM] (TESTS, testtools): Fix.

pipeconnect and socks5 should be changed if we run it on Windows. If
so, pipeconnect should use _open_osfhandle to get HANDLE for pipe.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>