Add python script that tests notebooks.

[grebe]

Address #32.

The test has a list of expected errors. The script runs each notebook and checks that it has the expected number of errors, and that each error contains a given substring.

[edwardcwang]

Nice! 💯

Two thoughts:

1. It seems to take quite long to run (>6 minutes and it didn't even finish; see below). I think we should add a flag to allow us to only run it for certain notebooks since it does take a while to run.

(e.g. this is what the Python3 language unit tests have - since it takes a while to run the entire test suite, it provides an option to only run parts of the test suite corresponding to the code you are actively working on.)

2. It seems to fail at module 3.3 for me for whatever reason...

[NbConvertApp] Converting notebook 3.3_higher-order_functions.ipynb to notebook
[NbConvertApp] Executing notebook with kernel: scala
[NbConvertApp] Writing 26512 bytes to /tmp/tmps3sx34x5.ipynb
3
5
Traceback (most recent call last):
  File "./runtest.py", line 69, in <module>
    check_errors(expected, errors)
  File "./runtest.py", line 35, in check_errors
    assert len(expected) == len(actual)
AssertionError

[grebe]

Re: 1, is there a canonical examples for this?

Re: 2, the test might be broken. I was getting timeouts on my current machine so I just pushed hoping that I had gotten it right earlier. It prints out the expected and actual number of errors, so the mismatch means I didn't collect all the errors.

Also, not sure if I'm going crazy, but I think I remembered seeing the number of errors vary from run to run. Keep an eye out for that.

[edwardcwang]

1. Just pushed a commit that implements something like that

2. It seems to be slow and failing on my machine (just ran it again):

[NbConvertApp] Converting notebook 3.3_higher-order_functions.ipynb to notebook
[NbConvertApp] Executing notebook with kernel: scala
[NbConvertApp] Writing 26512 bytes to /tmp/tmpywwuu19s.ipynb
3
5
Traceback (most recent call last):
  File "./runtest.py", line 82, in <module>
    check_errors(expected, errors)
  File "./runtest.py", line 38, in check_errors
    assert len(expected) == len(actual)
AssertionError

[grebe]

Working on fixes for the actual tests, it's just slow to run.

[chick]

This looks good to me.
I do get the following warnings when I run the tests
[NbConvertApp] WARNING | Timeout waiting for IOPub output
Should this be fixed before merging.

[grebe]

I think there is a way to change the timeout- not sure what it should be. I think we should add travis (or circleci- are we using that now?) and set the timeout so that whatever we use for CI is happy. I think that we should merge this an fix that up in a later PR adding CI support. If there are no objections, I'll merge.