Add new build system for Java tests

This commit adds a new Gradle subproject dedicated to Java tests.
Both kera_jni and kera_jni_test subprojects output their own shared
library, libkera_jni.so and libkera_jni_test.so, respectively. The
former links against libkera.so, whereas the latter links against
both libkera_jni.so and libkeratest.so.