LU-13981 tests: use $TRUNCATE consistently and correctly.

LU-13981 tests: use $TRUNCATE consistently and correctly.

Two changes:
 1/ tests should should always use $TRUNCATE rather than just
    "truncate" to ensure the tests/truncate programs is used
    rather than anything in /bin /usr/bin.  This might not
    be needed in practice, but it is good to be consistent.

 2/ The arguments provided should match what tests/truncate
    expects - "filename size".  Using "-s size filename"
    as expected by /usr/bin/truncate is confusing and
    error prone.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ic0adc0148ea0ff625695dbfc4cf339d0eca70241
Reviewed-on: https://review.whamcloud.com/40005
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>