test: re-bundle cat on windows (#21255)

The builtin cat was removed in 4bc9229ecb
as it is not used during runtime but only for tests. However, it is a
very small and useful utility program that we need for a lot of our
tests, so there's no harm in bundling it, and it helps us avoid
complicating our build system by having two versions of neovim (neovim
for users and neovim for testing).

Also skip tests if "grep" or "sleep" isn't available.

test/functional/provider/perl_spec.lua

@@ -9,8 +9,6 @@ local curbufmeths = helpers.curbufmeths
 local insert = helpers.insert
 local expect = helpers.expect
 local feed = helpers.feed
-local is_os = helpers.is_os
-local skip = helpers.skip
 
 do
   clear()
@@ -26,8 +24,6 @@ before_each(function()
 end)
 
 describe('legacy perl provider', function()
-  skip(is_os('win'))
-
   it('feature test', function()
     eq(1, eval('has("perl")'))
   end)
@@ -70,7 +66,6 @@ describe('legacy perl provider', function()
 end)
 
 describe('perl provider', function()
-  skip(is_os('win'))
   teardown(function ()
     os.remove('Xtest-perl-hello.pl')
     os.remove('Xtest-perl-hello-plugin.pl')