LU-15740 tests: add more stats to runtests

LU-15740 tests: add more stats to runtests

Print out the space usage at the start, middle, and end of
runtests, so that it is easier to see where the space is
going, and how much is used at peak consumption.  The goal
of the test is to avoid space leakage in object create and
destroy, but it is OK if there is some usage for internal
files like llogs, quotas, etc.

Move the "mkdirmany" call to the first phase, add a statmany
to the middle phase, and "rmdirmany" to the end so that it
is also checking the directory validity after a remount.

Update createmany and statmany to be a bit easier to use.

Test-Parameters: trivial testlist=runtests env=SLOW=yes mdscount=2 mdtcount=4
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iec8cb56501c7e75b620951a2d669b0dd6bb0a36f
Reviewed-on: https://review.whamcloud.com/47065
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: cliff white <cwhite@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>