Bug 18736: (QA follow-up) Change to signed, add large int test

Bug 18736: (QA follow-up) Change to signed, add large int test

[1] Add trivial subtest in t/Number/Price.t
Checking a negative number and a large number for the MAX_INT change.
Note: Confusing to have t/Prices.t too.

[2] Change UNSIGNED to SIGNED in get_rounding_sql. Although I did not spot
problems with negative prices, we theoretically could while casting.
cast(-2 as unsigned) == 18446744073709551614

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>