add test

test/samples/docs/doc_1.txt

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+Some text for testing.
+Two lines in here.

test/samples/docs/doc_2.txt

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+A Doc specifically talking about haystack.
+Haystack can be used to scale QA models to large document collections.

test/test_db.py

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+from haystack.indexing.io import write_documents_to_db, fetch_archive_from_http
+from haystack.database.orm import Document
+
+
+def test_db_write_read():
+    from haystack.database import db
+    db.drop_all()
+    db.create_all()
+
+    write_documents_to_db(document_dir="samples/docs")
+    documents = db.session.query(Document).all()
+    assert len(documents) == 2
+    assert documents[0].text == 'A Doc specifically talking about haystack.\nHaystack can be used to scale QA models to large document collections.'
+    print(documents)