Centralize logic for transactional sink tests

[olukas]

There is some logic which is similar for transactional sink tests (in modules snapshot-file-test, snapshot-jdbc-test and snapshot-jms-sink-test).

1. Tests for transactional sinks use source as following:

StreamSource<Integer> source = SourceBuilder
        .stream(NAME, procCtx -> new int[1])
        .<Integer>fillBufferFn((ctx, buf) -> {
            buf.add(ctx[0]++);
            sleepMillis(SLEEP);
        })
        .createSnapshotFn(ctx -> ctx[0])
        .restoreSnapshotFn((ctx, state) -> ctx[0] = state.get(0))
        .build();

2. Verifiers JmsSinkVerifier, GeneratedFilesVerifier and JdbcSinkVerifier use the similar logic.

It should be centralized.