Bug 40051: Fix test after change of default value for compress_tar

Default value for compress_tar was changed in
a91d4653.

test/projects/mozmill-automation/config

 version: '[% c("abbrev") %]'
 hg_url: https://hg.mozilla.org/qa/mozmill-automation/
 hg_hash: bbad7215c713
+compress_tar: ''
 t: '[% sha256(exec("cat testrun_remote.py")) %]'
 build: |
   #!/bin/sh