Improve FFI test suite for Windows

We add Windows support to four existing test cases, extract some useful
utility functions, and use them to simplify further test cases.

We als

Improve FFI test suite for Windows

We add Windows support to four existing test cases, extract some useful
utility functions, and use them to simplify further test cases.

We also remove the Windows specific code from preload.inc, since
preloading isn't supported on Windows anyway.

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