test_all: mark linkbox fs/sync test TestSyncOverlapWithFilter as ignore

This gives the error

> Update second step failed: Linkbox error 500: The file name needs to include a suffix, such as xxx.mp4

As linkbox can't have files starting with "." and we are trying to save a file called ".ignore".

Author: ncw

fstest/test_all/config.yaml

@@ -425,6 +425,7 @@ backends:
      - TestCaseInsensitiveMoveFile
      - TestFixCase
      - TestListDirSorted        # Can't upload files starting with . - FIXME fix with encoding
+     - TestSyncOverlapWithFilter # Can't upload files starting with . - FIXME fix with encoding
  - backend:  "premiumizeme"
    remote:   "TestPremiumizeMe:"
    fastlist: false