* Fix createSuper in Babel <7.5.5
In Babel <7.5.5, addHelper only throws when called for the first time.
This was fixed by babel/babel#10208, but since we are already adding
a workaround for <7.9.0, we may as well fix <7.5.5
* Use 7.0.0 in "babel-old-version" test
* Test Babel compiled using an old version of @babel/core
This should catch regressions where we use a new helper without a fallback
for older versions, or when we use a new @babel/traverse method.
* Enable the test