testsuite: use shell arithmetic instead of fork-execing expr
The tessuite use a few counters to keep track of things.
These counters are incremented via the 'expr' command.
But this command is not a shell builtin and need thus
to be fork+execed which need more CPU ressources than
using some builtin operation.
Change this by doing all the arithmetic via the portable
'$(( ... ))' shell builtin.
This speedup the testsuite by another 10%.
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>