add '--numprocesses' as long option for '-n'

This is more verbose in scripts that run pytest-xdist. It can also help
in some ci-helper scripts like to auto detect if  pytest-xdist should be
installed.

testing/test_plugin.py

@@ -20,6 +20,10 @@ def test_dist_options(testdir):
     check_options(config)
     assert config.option.dist == "load"
     assert config.option.tx == ['popen'] * 2
+    config = testdir.parseconfigure("--numprocesses 2")
+    check_options(config)
+    assert config.option.dist == "load"
+    assert config.option.tx == ['popen'] * 2
     config = testdir.parseconfigure("-d")
     check_options(config)
     assert config.option.dist == "load"