implement component tests

[bcliang]

boilerplate provides an example of test run using pytest. Let's implement tests for the following:

ExtendData input callback for a new ExtendableGraph object (no props) should revert to using this.plot()

• check layoutis default
• check figureis default
• check number of traces matches len(extendData)

ExtendData input callback (n-count traces) for an ExtendableGraph object with existing (n-count) traces should extend traces as expected.

• check len(figure.data[0...n].[x, y])

ExtendData input callback (n-count traces) for an ExtendableGraph object with existing (m-count, m>n) traces should extend-then-append traces.

• check len(figure.data[n+1...m].[x, y])

[bcliang]

3 tests added. Removed test_usage.py.