tests: make ipcs limit calculations work when numbers are large

Test suite  failed when I ran it with kernel 3.16.  Error is the
following.

tests/ts/ipcs/limits2: line 31: [: 18446744073692774399: integer expression expected

Reference: http://lkml.iu.edu/hypermail/linux/kernel/1406.0/01869.html
Signed-off-by: Sami Kerola <kerolasa@iki.fi>