software/slapos-testing: Pin zc.lockfile for KeDiFa

Needed for nexedi/kedifa!2

/reviewed-on nexedi/slapos!537

software/slapos-testing/software.cfg

@@ -187,6 +187,7 @@ Pygments = 2.1.3
 slapos.recipe.template = 4.3
 
 # All depencies should be pinned.
+zc.lockfile = 1.4
 bcrypt = 3.1.4
 dnspython = 1.15.0
 funcsigs = 1.0.2