Tests: better binary path handling on Windows. The ".exe" extension is no longer required, and testing is allowed if it is omitted. Additionally, forward slashes in the binary path are automatically replaced with reverse slashes, since CMD cannot properly handle relative paths with forward slashes, and such paths previously resulted in various issues, including non-working $t->has() and $t->has_version(). In particular, with these changes the default binary path, which is "../nginx/objs/nginx", works properly, and testing can be done without any additional options.