(Maint) Don't assume eventlog gem is installed on Windows

Previously, the test would fail when run on a Windows box that didn't
have the eventlog gem installed. Since the Windows agent should be able
to run without the gem installed, and fall back to writing to a log
file, this commit changes the test to only run when the gem is
installed. There is already a test that verifies that we fall back if
the eventlog feature is not available.