tests: fix false du failure on newer XFS

On XFS, when creating the ~2G test file 'big' in a for-loop by
appending 20M each time, the file ends up using ~4G - visible in
'st_blocks'.  The unused space would be reclaimed later.
This feature is called "speculative preallocation" which aims at
avoiding fragmentation.
According to the XFS FAQ [1], there are two particular aspects of
XFS speculative preallocation that are triggering this:

  1. "Applications that repeatedly trigger preallocation and reclaim
     cycles [after file close] can cause fragmentation.
     Therefore, this pattern is detected and causes the preallocation
     to persist beyond the lifecycle of the file descriptor."

  2. "Preallocation sizes grow as files grow larger."

[1] http://xfs.org/index.php/XFS_FAQ

Avoid one of the above by only doing a single close (reclaim cycle).

* tests/du/2g.sh: Similar to the fix for a dd test (see commit
v8.22-65-g7c03fe2), avoid speculative preallocation by creating
the 'big' file in one go instead of appending to it in the loop.
Remove debugging statements as the output with 'set -x' is
sufficient nowadays.

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