Fix a typo in the Net::Ping::External test

This typo had the effect of setting the expected
ping duration to 5.5s instead of 1.5s. Since in Windows
the defaul ping timeout is 4s, the test always passed.
Avoid implementing timeout on Windows

The -w option of ping on Windows has the same effect
of an external timeout.
Merge branch 'feature/fix-win32-external-ping' into develop