On Windows, no echo.exe exists normally, so calling echo from the tests
is no good idea, since it relies on Cygwin to be installed and in %PATH%
(so that echo.exe is available).
This fixes this by providing a small echo.bat which is callable from the
tests, and then using a platform-specific path to the executable instead
of the hardcoded "echo". This should ensure that the tests pass even on
systems where echo.exe is not installed.
Note that we can't simply use a do-nothing exe (like rundll or hh.exe),
as we're passing parameters, and those executables may behave
differently in the presence of those parameters.
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