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/ui/cmdline_spec.lua

@@ -4,10 +4,10 @@ local clear, feed = helpers.clear, helpers.feed
 local source = helpers.source
 local command = helpers.command
 local assert_alive = helpers.assert_alive
-local uname = helpers.uname
 local exec = helpers.exec
 local eval = helpers.eval
 local eq = helpers.eq
+local is_os = helpers.is_os
 
 local function new_screen(opt)
   local screen = Screen.new(25, 5)
@@ -717,7 +717,7 @@ describe('cmdline redraw', function()
   end)
 
   it('with <Cmd>', function()
-    if string.find(uname(), 'bsd') then
+    if is_os('bsd') then
       pending('FIXME #10804')
     end
     command('cmap a <Cmd>call sin(0)<CR>')  -- no-op