Test a few very basic defragmentation scenarios.

Test a few very basic defragmentation scenarios.

This test creates some files, runs defrag on them,
and compares the before/after fragmentation as well
as file md5sums and timestamps.

The test currently expects to find e4defrag in
/usr/bin

It should be relatively easy to add more interestingly
fragmented files to the tests, as well as to test
that memory-mapped files aren't touched, etc -
but this gives us a framework.

V2: remount before checking file contents, and create
common.defrag.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>