Fix tests on Windows

* Skip problematic gems on Windows
* Skip tests for features unavailable in Windows
* Fix checks for external commands to work in Windows
* etc.

This changes test statistics on my Windows 7 machine from
`612 tests, 1602 assertions, 2 failures, 18 errors, 12 skips` to `612 tests, 1601 assertions, 1 failures, 2 errors, 29 skips`