Move all unit tests to their modules and remove briar-tests

bramble-api/build.gradle

...st/java/org/briarproject/bramble/BrambleMockTestCase.java

...c/test/java/org/briarproject/bramble/BrambleTestCase.java

...test/java/org/briarproject/bramble/ImmediateExecutor.java

...est/java/org/briarproject/bramble/TestDatabaseConfig.java

...est/java/org/briarproject/bramble/TestDatabaseModule.java

.../test/java/org/briarproject/bramble/TestSeedProvider.java

...java/org/briarproject/bramble/TestSeedProviderModule.java

...api/src/test/java/org/briarproject/bramble/TestUtils.java

...test/java/org/briarproject/bramble/ValidatorTestCase.java

.../org/briarproject/bramble/api/data/BdfDictionaryTest.java

...t/java/org/briarproject/bramble/api/data/BdfListTest.java

bramble-core/.gitignore

bramble-core/build.gradle

...ore/src/test/java/org/briarproject/bramble/RunAction.java

.../briarproject/bramble/client/BdfMessageValidatorTest.java

...org/briarproject/bramble/client/ClientHelperImplTest.java

.../briarproject/bramble/contact/ContactManagerImplTest.java

...java/org/briarproject/bramble/crypto/AsciiArmourTest.java

...va/org/briarproject/bramble/crypto/Blake2sDigestTest.java