Extract some code from yaml test item (#149)

* Make this function return an int

Prior to this a SystemExit exception could return the code as a string
or an integer, now we convert it to a sensible integer to return

* Move where the extension hook is executed

It currently doesn't depend on the execution path existing, and we can
make sure this runs before we create that path and run anything

* Extract code to check mypy output

This is so we can abstract away what is involved in checking the mypy
output. This will also help when we extract running mypy so we don't
have to pass around the information that is only needed for this
check

* Extract out code for running mypy

This will make it easier for us to run mypy multiple times later on

* Extract the running of the test item

This object glues together the creation of test files, the running of
mypy, and the checking of the result

* Remove unused imports