Generate non-test JAR for netty-testsuite

Motivation:

So far, we generated and deployed test JARs to Maven repositories. The
deployed JAR had the classifier 'test-jar'.  The test JAR is consumed by
transport-native-epoll as a test dependency.

The problem is, when netty-transport-native-epoll pulls the test JAR as
a dependency, that Maven resolves its transitive dependencies at
'compile' and 'runtime' scope only, which is incorrect.

I was bitten by this problem recently while trying to add a new
dependency to netty-testsuite.  Because I added a new dependency at the
'test' scope, the new dependency was not pulled transitively by
transport-native-epoll and caused an unexpected build failure.

- d6160208c37cfd21b690f1a0df67776bf986b28e
- bf77bb4c3ae04778e65b8a38c2a9ec0e15207de8

Modifications:

- Move all classes in netty-testsuite from src/test to src/main
- Update the 'compile' scope dependencies of netty-testsuite
- Override the test directory configuration properties of the surefire
  plugin
- Do not generate the test JAR anymore
- Update the dependency of netty-transport-native-epoll

Result:

It is less error-prone to add a new dependency to netty-testsuite.