component/*/test*cfg: reorganise now that python3 is default

test.cfg becomes the python3 version
a new test-py2.cfg is introduced
test-py3.cfg is removed

cf slapos commit cc7f36a0 (component/defaults: use python3 by default, 2022-08-27)

component/nxdtest/test-py3.cfg → component/nxdtest/test-py2.cfg

@@ -5,4 +5,4 @@ extends =
   test.cfg
 
 [python]
-part = python3
+part = python2.7

component/pygolang/test-py3.cfg → component/pygolang/test-py2.cfg

-# SlapOS software release to test pygolang/py3 on Nexedi testing infrastructure.
+# SlapOS software release to test pygolang/py2 on Nexedi testing infrastructure.
 
 [buildout]
 extends = test.cfg
 
 [python]
-part = python3
+part = python2.7