test: simplify platform detection (#21020)

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

test/functional/preload.lua

@@ -2,10 +2,10 @@
 -- Busted started doing this to help provide more isolation.  See issue #62
 -- for more information about this.
 local helpers = require('test.functional.helpers')(nil)
-local iswin = helpers.iswin
 local busted = require("busted")
+local is_os = helpers.is_os
 
-if iswin() then
+if is_os('win') then
   local ffi = require('ffi')
   ffi.cdef[[
   typedef int errno_t;