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/lua/fs_spec.lua

@@ -7,10 +7,10 @@ local mkdir_p = helpers.mkdir_p
 local rmdir = helpers.rmdir
 local nvim_dir = helpers.nvim_dir
 local test_build_dir = helpers.test_build_dir
-local iswin = helpers.iswin
 local nvim_prog = helpers.nvim_prog
+local is_os = helpers.is_os
 
-local nvim_prog_basename = iswin() and 'nvim.exe' or 'nvim'
+local nvim_prog_basename = is_os('win') and 'nvim.exe' or 'nvim'
 
 before_each(clear)
 
@@ -102,7 +102,7 @@ describe('vim.fs', function()
       eq('C:/Users/jdoe', exec_lua [[ return vim.fs.normalize('C:\\Users\\jdoe') ]])
     end)
     it('works with ~', function()
-      if iswin() then
+      if is_os('win') then
         pending([[$HOME does not exist on Windows ¯\_(ツ)_/¯]])
       end
       eq(os.getenv('HOME') .. '/src/foo', exec_lua [[ return vim.fs.normalize('~/src/foo') ]])