software/slapos-testing: use slapos.core[test]

extra_requires was added in slapos.core, which simplifies this part.

software/slapos-testing/software.cfg

@@ -61,8 +61,7 @@ depends = ${slapos.core-setup:egg}
 
 [slapos.core-setup]
 <= setup-develop-egg
-# XXX slapos.core does not have `test` extra require, `mock`, `pyflakes` and `httmock` are only listed in `tests_require` and are listed explicitly
-egg = slapos.core
+egg = slapos.core[test]
 setup = ${slapos.core-repository:location}
 
 [slapos.recipe.build-setup]
@@ -116,8 +115,6 @@ eggs =
   ${slapos.rebootstrap-setup:egg}
   mock
   zope.testing
-  httmock
-  pyflakes
   supervisor
 entry-points =
   runTestSuite=erp5.util.testsuite:runTestSuite