Handle missing space with -p

This still does not use an actual argument parser, which only gets
instantiated below, and it does not appear to make sense instantiating
it just for this pre-parsing it seems.

`-p` without the required value is being handled before already though,
so it could potentially be passed down from somewhere already?!

Fixes https://github.com/pytest-dev/pytest/issues/3532.

testing/test_pluginmanager.py

@@ -323,6 +323,12 @@ class TestPytestPluginManagerBootstrapming(object):
             ImportError, lambda: pytestpm.consider_preparse(["xyz", "-p", "hello123"])
         )
 
+        # Handles -p without space (#3532).
+        with pytest.raises(ImportError) as excinfo:
+            pytestpm.consider_preparse(["-phello123"])
+        assert '"hello123"' in excinfo.value.args[0]
+        pytestpm.consider_preparse(["-pno:hello123"])
+
     def test_plugin_prevent_register(self, pytestpm):
         pytestpm.consider_preparse(["xyz", "-p", "no:abc"])
         l1 = pytestpm.get_plugins()