Reset entire test db before each test

[dennisvang]

Many tests in the current implementation rely on cleaning up (parts of) the db beforehand, for example

FAIRDataPoint/src/test/java/org/fairdatapoint/acceptance/index/admin/List_Trigger_POST.java

Lines 159 to 160 in f65cd35

	indexEntryRepository.deleteAll();
	indexSettingsRepository.deleteAll();

Cleaning up the db before each test is a valid strategy, see e.g. here, but the current implementation only cleans up specific repositories, instead of resetting the entire db.
As a result, it is easy to forget to clean one up, which may lead to nasty surprises later on, such as #839.

Note that an alternative would be to mark tests with @DataJpaTest or @Transactional, which would roll back any changes after the test. Also see #816

[dennisvang]

For the support/v1.18.x branch (which uses mongodb), this could be "fixed" by adding more "development migrations" here:

FAIRDataPoint/src/main/java/nl/dtls/fairdatapoint/database/mongo/migration/development/MigrationRunner.java

Line 102 in 6e98961

indexEntryMigration.runMigration();

For example, there is no migration for the index settings repository (probably the ultimate cause of #839), and perhaps more are missing.

These dev migrations start by cleaning the repository:

FAIRDataPoint/src/main/java/nl/dtls/fairdatapoint/database/mongo/migration/development/index/entry/IndexEntryMigration.java

Lines 40 to 41 in 6e98961

	public void runMigration() {
	indexEntryRepository.deleteAll();

[dennisvang]

For the develop branch (which uses PostgreSQL), it looks like this is already covered here:

FAIRDataPoint/src/test/java/org/fairdatapoint/WebIntegrationTest.java

Lines 68 to 72 in f65cd35

	public void setup() {
	flyway.clean();
	flyway.migrate();
	aclMigration.runMigration();
	}

So maybe we can simply remove the *Repository.deleteAll() calls from the individual test methods?