Add alternative of gatekeeper test on Windows

Thank you for your [ROpenSci community call presentation](https://ropensci.org/commcalls/2019-05-07/) 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`](https://github.com/ropenscilabs/defender/issues/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](https://github.com/ropenscilabs/ropsec/blob/master/inst/tests/simple/macos-simple-test.R) in [`os_type` or a new `is_macos`](https://github.com/ropenscilabs/ropsec/blob/e21902f07563f10d856335518474e7e1da0415a5/R/utils.R#L13-L19)
2. add tests for [whatever is Windows Gatekeeper-equivalent](https://www.theregister.co.uk/2017/02/28/microsoft_restricts_windows_software/)

**Additional context**

Start with inst/tests/simple/macos-simple-test.R and the os_type or is_macos helpers discussed in R/utils.R. Run ropsec::summarize_system_checks() on a Windows environment to reproduce the failure, then verify that the macOS Gatekeeper test is skipped there while remaining runnable on macOS.

Written by the indexing model from the issue text.