test/ec: Fix spurious failures caused by self-heal

The sha1sum of a file may update the access time of that file.
If this happens while a brick is down, as it is forced in the
test, that brick doesn't get the update, getting out of sync.

When the brick is restarted, self-heal repairs the file, but
the test shouldn't access brick contents until self-heal finishes.
If this is combined with a kill of another brick before self-heal
has finished repairing the file, the volume could become inaccessible.

Since the purpose of these tests is only to check ec functionality
(there is another test that checks self-heal), the test that corrupts
the file has been removed.

Additional checks to validate the state of the volume have been added
to avoid some timing issues.

This is a backport of http://review.gluster.org/8892/

BUG: 1149118
Change-Id: I8a40b7f07fc8ecd2c721bad1bcdd351dd8504155
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/8902
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Dan Lambright <dlambrig@redhat.com>