selftests: tc-testing: u32: Add tests covering sample option

Kernel's key folding basically consists of shifting away least
significant zero bits in mask and masking the resulting value with
(divisor - 1). Test for u32's 'sample' option to behave identical.

Suggested-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>