Add alternative of gatekeeper test on Windows

[katrinleinweber]

Thank you for your ROpenSci community call presentation recently :-)

Is your feature request related to a problem? Please describe.
I ran ropsec::summarize_system_checks() on Windows 8 and got

macos-simple-test.R:45: error: gatekeeer is on
'spctl' not found
1: system("spctl --status", intern = TRUE) at C:/.../R/win-library/3.6/ropsec/tests/simple/macos-simple-test.R:45

which is no surprise. Same sessionInfo() as in defender/#20.

Describe the solution you'd like

This test should be skipped on Windows systems.

Describe alternatives you've considered

1. wrap macos-simple-test.R in os_type or a new is_macos
2. add tests for whatever is Windows Gatekeeper-equivalent

Additional context

[czeildi]

Thanks! If you'd like, you are very welcome to submit a PR.

[czeildi]

For now I suppressed these tests with e61ece1 but did not yet provide an alternative. I'll edit the issue accordingly