objectstore/store_test.cc: fix unintentional integer overflow

CID 1232603 (#1 of 1): Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
 overflow_before_widen: Potentially overflowing expression
 objs_per_folder * folders_range(rng) with type int (32 bits,
 signed) is evaluated using 32-bit arithmetic before being
 used in a context which expects an expression of type
 uint64_t (64 bits, unsigned). To avoid overflow, cast
 either operand to uint64_t before performing the multiplication.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>