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/api/server_notifications_spec.lua

@@ -5,7 +5,7 @@ local eq, clear, eval, command, nvim, next_msg =
 local meths = helpers.meths
 local exec_lua = helpers.exec_lua
 local retry = helpers.retry
-local isCI = helpers.isCI
+local is_ci = helpers.is_ci
 local assert_alive = helpers.assert_alive
 local skip = helpers.skip
 
@@ -79,7 +79,7 @@ describe('notify', function()
   end)
 
   it('cancels stale events on channel close', function()
-    skip(isCI(), 'hangs on CI #14083 #15251')
+    skip(is_ci(), 'hangs on CI #14083 #15251')
     local catchan = eval("jobstart(['cat'], {'rpc': v:true})")
     local catpath = eval('exepath("cat")')
     eq({id=catchan, argv={catpath}, stream='job', mode='rpc', client = {}}, exec_lua ([[