Follows up 7552f939c6 "tests: disable most gpg integration tests on Windows."
I couldn't find this test failing in a few samples before, but it does now.
This adds a guard to the gpg signing tests which will skip the test if
`gpg` is not installed on the system.
This is done in order to avoid requiring all collaborators to have setup
all the tools on their local machines that are required to test commit
signing.
