In particular, `IsSanitizerCharacteristic` is a negative _source_ characteristic (not a negative sink characteristic), while `NeutralModelCharacteristic` is both.
This eliminates the erroneous test results.
Instead of positively implying the negative sink type, negative sink characteristics now negatively imply all sink types (but not source types). This is simpler and sice we will never have a huge number of sink types it doesn't impact performance either.
Changes to test results:
- The call to `createDirectories` at `Test.java:87` is now correctly classified as a source candidate, having previously been erroneously excluded by a negative _sink_ characteristic.
- The call to `compareTo` at `Test.java:48` is now erroneously classified as a source candidate; it should be suppressed by `IsSanitizerCharacteristic`, which is a negative sink characteristic, but should really be a negative source characteristic.
- In framework mode, several endpoints are now erroneously classified as source candidates even though they have neutral models, because `NeutralModelCharacteristic` is currently only a negative sink characteristic and not a negative source characteristic.
