4667.	[cleanup]	Refactor RDATA unit tests. [RT #45610]

9.9.12, 9.10.7, 9.11.3, 9.12.0

Cherry-picking to v9_9 and v9_9_sub was done without updating the
"source generic" test and with the expected result for EDNS Client
Subnet test "Option code family 0, source 0, scope 0" changed to
ISC_FALSE.  This is because the v9_9 branch lacks change 4468 ("Address
ECS option handling issues. [RT #43191]") which changes the way the
relevant wire data is processed.  Change 4468 is present in all of
v9_10, v9_10_sub and v9_11.