Fix build

[chris-ch]

I had the tests failing with the "next" branch:
@chris-ch ➜ /workspaces/fred (next) $ ./gradlew --parallel test

Task :compileTestJava FAILED
/workspaces/fred/test/freenet/client/filter/TheoraBitstreamFilterTest.java:12: error: cannot find symbol
import static org.junit.Assert.assertThrows;
^
symbol: static assertThrows
location: class Assert
/workspaces/fred/test/freenet/support/api/BooleanCallbackTest.java:4: error: cannot find symbol
import static org.junit.Assert.assertThrows;
^
symbol: static assertThrows
location: class Assert
/workspaces/fred/test/freenet/support/SimpleFieldSetTest.java:20: error: cannot find symbol
import static org.hamcrest.Matchers.anEmptyMap;
^
symbol: static anEmptyMap
location: class Matchers
...

I needed to upgrade junit to 4.13.2 and hamcrest to 3.0 ... The process requires updating the verification keys.