fix: correct failing repo test

The previous version grepped for the number 0, which is now output in the repo cache line count. We should check for the exact value instead.

tests/unit/repo.bats

@@ -40,18 +40,20 @@ teardown() {
   echo "status: $status"
   assert_success
 
-  run /bin/bash -c "find $DOKKU_ROOT/$TEST_APP/cache -type f | wc -l | grep 0"
-  echo "output: $output"
+  run /bin/bash -c "find $DOKKU_ROOT/$TEST_APP/cache -type f | wc -l"
+  echo "count: '$output'"
   echo "status: $status"
-  assert_failure
+  assert_not_output 0
+
   run /bin/bash -c "dokku repo:purge-cache $TEST_APP"
   echo "output: $output"
   echo "status: $status"
   assert_success
-  run /bin/bash -c "find $DOKKU_ROOT/$TEST_APP/cache -type f | wc -l | grep 0"
-  echo "output: $output"
+
+  run /bin/bash -c "find $DOKKU_ROOT/$TEST_APP/cache -type f | wc -l"
+  echo "count: '$output'"
   echo "status: $status"
-  assert_success
+  assert_output 0
 
   run destroy_app
   echo "output: $output"