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/treesitter/parser_spec.lua

@@ -5,7 +5,7 @@ local eq = helpers.eq
 local insert = helpers.insert
 local exec_lua = helpers.exec_lua
 local feed = helpers.feed
-local iswin = helpers.iswin
+local is_os = helpers.is_os
 local skip = helpers.skip
 
 before_each(clear)
@@ -684,7 +684,7 @@ int x = INT_MAX;
       end)
 
       it("should not inject bad languages", function()
-        skip(iswin())
+        skip(is_os('win'))
         exec_lua([=[
         vim.treesitter.add_directive("inject-bad!", function(match, _, _, pred, metadata)
           metadata.language = "{"