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/vimscript/execute_spec.lua

@@ -8,7 +8,7 @@ local funcs = helpers.funcs
 local Screen = require('test.functional.ui.screen')
 local command = helpers.command
 local feed = helpers.feed
-local iswin = helpers.iswin
+local is_os = helpers.is_os
 
 describe('execute()', function()
   before_each(clear)
@@ -265,7 +265,7 @@ describe('execute()', function()
   -- This deviates from vim behavior, but is consistent
   -- with how nvim currently displays the output.
   it('captures shell-command output', function()
-    local win_lf = iswin() and '\13' or ''
+    local win_lf = is_os('win') and '\13' or ''
     eq('\n:!echo foo\r\n\nfoo'..win_lf..'\n', funcs.execute('!echo foo'))
   end)