Improve test coverage

...otocol/serializers/handshake/SessionInitSerializerTest.kt

.../protocol/serializers/quassel/IrcChannelSerializerTest.kt

...sel/protocol/serializers/quassel/IrcUserSerializerTest.kt

...rotocol/serializers/signalproxy/InitDataSerializerTest.kt

...el/protocol/serializers/signalproxy/SyncSerializerTest.kt

...stjanne/libquassel/protocol/syncables/AliasManagerTest.kt

...nne/libquassel/protocol/syncables/BufferViewConfigTest.kt

...ne/libquassel/protocol/syncables/BufferViewManagerTest.kt

...libquassel/protocol/syncables/HighlightRuleManagerTest.kt

...ne/libquassel/protocol/syncables/IgnoreListManagerTest.kt

...justjanne/libquassel/protocol/syncables/IrcChannelTest.kt

...de/justjanne/libquassel/protocol/syncables/IrcUserTest.kt

...de/justjanne/libquassel/protocol/syncables/NetworkTest.kt

...nne/libquassel/protocol/syncables/invokers/InvokerTest.kt

.../de/justjanne/libquassel/protocol/testutil/MockSession.kt

...otlin/de/justjanne/libquassel/protocol/testutil/Random.kt

...tjanne/libquassel/protocol/testutil/mocks/EmptySession.kt

...ne/libquassel/protocol/testutil/mocks/RealisticSession.kt