tc-testing: updated pedit TDC tests

Added tests for u8/u32 clear value, u8/16 retain value, u16/32 invert value,
u8/u16/u32 preserve value and test for negative offsets.

Signed-off-by: Roman Mashak <mrv@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>