add empty tests

What do you think about just using the built in unittest module?

For now this just adds a skipped test for each module.  If this is approved, I'll start adding actual tests and hopefully others will join in.

```
src/electrum $ python -m unittest discover
ssssssssssssssssssssssssssssssssssssssssssssssssss
----------------------------------------------------------------------
Ran 50 tests in 0.002s

OK (skipped=50)