LU-15316 tests: use integers in sanity test_255a

LU-15316 tests: use integers in sanity test_255a

The [[ ... > ... ]] operator doesn't really compare floats, it
compares strings.  That works as expected if the strings are
the same length, but fails for comparisons like [[ 32 > 123 ]].
Use (( ... > ... )) for comparisons, and only use integer values.

This test has been failing intermittently forever, but the error
was ignored because of running in a VM.

Test-Parameters: trivial
Fixes: f3b8f3fad502 ("tests: fix float comparison in sanity test_255a")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I6787082cd579ae3f1bdd43222a739c939d3ebbe5