Add signalling so tests know when yield-generator has started

[chris-belcher]

One issue on some machines is the test suite has a hardcoded wait time after creating a yield-generator.
It would be better to have the yield generator signal to the test suite that it's ready.

Here is one way to do this:
When yield-generator is ready it does the equivalent of touch logs/yigen-statement.csv.
The test suite simply loops and waits for the last modified time to be updated.

[chris-belcher]

A problem with this is it only works for one yield generator. Maybe use another file instead of yigen-statement.csv, one file for each bot.