Fix TestPGMinimize0 unittest
Fix TestPGMinimize0 unittest

This test have been failing after e22f9ee, however the failure is ignored
by CI.

plutoshe/taskgraph,dzch/taskgraph,kod3r/taskgraph,dzch/taskgraph,kod3r/taskgraph,taskgraph/taskgraph,taskgraph/taskgraph,fengjingchao/taskgraph,fengjingchao/taskgraph,5kg/taskgraph,plutoshe/taskgraph,5kg/taskgraph