tests: Fix bug in test_snapshot_goal

This test was failing from time to time (especially when using
valgrind) because of the assertion:

  mfssetgoal 4 file
  FILE_SIZE=$((1000 + LIZARDFS_CHUNK_SIZE)) file-generate file
  assert_equals 8 $(find_all_chunks | wc -l)

This condition wasn't true when chunks were rebalanced. This commit
fixes the test by disabling chunk rebalancing at all by setting
ACCEPTABLE_DIFFERENCE to the maximum possible value.

Change-Id: Ibfd67751df0cb8c963aac6e4cb3b436d1d399143