win: test: enable job_spec.lua

- Default to powershell.
- Avoid hardcoded "-c".
- Remove ^M character from received lines.
- pending_win32(): clear() is unnecessary and it pollutes the tests.

Closes #3973
Helped-by: Rui Abreu Ferreira <raf-ep@gmx.com>

test/helpers.lua

@@ -68,6 +68,7 @@ local uname = (function()
     local status, f = pcall(io.popen, "uname -s")
     if status then
       platform = f:read("*l")
+      f:close()
     else
       platform = 'Windows'
     end