for task up to date check both status and sources (#1035)

* remove redundant if statement

* add subtests to TestStatusChecksum

testdata/checksum/Taskfile.yml

@@ -10,3 +10,12 @@ tasks:
     generates:
       - ./generated.txt
     method: checksum
+
+  build-with-status:
+    cmds:
+      - cp ./source.txt ./generated.txt
+    sources:
+      - ./source.txt
+    status:
+      - test -f ./generated.txt
+