LU-17527 tests: fix syntax error in test_255a

LU-17527 tests: fix syntax error in test_255a

The syntax error comes from the fact that the average speed can be less
than 1 (e.g. .85) which means that ${average_cache%.*} is actually empty
which means that the left parameter of < is empty.

This patch fixes the test by using the speedup instead. The test should
compare speedup_cache and speedup_ladvise with lowest_speedup instead of the
average read time.

Test-Parameters: trivial testlist=sanity env=ONLY=255a,ONLY_REPEAT=50
Signed-off-by: Courrier Guillaume <guillaume.courrier@cea.fr>
Change-Id: Ie2cd24f813a0efe65e3391a3fb664b9db39a9f92
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54009
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>