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/ex_cmds/source_spec.lua

@@ -14,9 +14,9 @@ local eval = helpers.eval
 local exec_capture = helpers.exec_capture
 local neq = helpers.neq
 local matches = helpers.matches
-local iswin = helpers.iswin
 local mkdir = helpers.mkdir
 local rmdir = helpers.rmdir
+local is_os = helpers.is_os
 
 describe(':source', function()
   before_each(function()
@@ -44,7 +44,7 @@ describe(':source', function()
   end)
 
   it("changing 'shellslash' changes the result of expand()", function()
-    if not iswin() then
+    if not is_os('win') then
       pending("'shellslash' only works on Windows")
       return
     end