Add DemoDB() initialization in setUp()

I added DemoDB() initialization in setUp() for tests and the tests were passing (except for one). But there are two issues/observations:

1) setUp() method is run before every test method. So Refresh schema and Indexing schema happens at every test method. This could be a good thing. But if you think it is inefficient, there may be a way such that the DemoDB initialization happens only once before all the tests are run. If you want that, I can modify it. 

2) The test test_table_keys_per_column fails and I get the following error:

======================================================================
ERROR: test_table_keys_per_column (tests.PandaSQLTest)
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Traceback (most recent call last):
  File "/Users/ue/Desktop/Projects/db.py/db/tests/tests.py", line 73, in test_table_keys_per_column
    short_db = DemoDB(keys_per_column=1)
TypeError: DemoDB() takes no arguments (1 given)

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Ran 14 tests in 0.127s

FAILED (errors=1)

Did you plan to add default parameter to DemoDB?