test: Use the default Jest preset
test: Use the default Jest preset

With this setting, ts-jest will take care of .ts and .tsx files only, leaving JavaScript files as-is.

This means we don't have to tell ts-jest to skipBabel anymore.

See https://kulshekhar.github.io/ts-jest/user/config/#the-3-presets

bkeepers/PRobot,probot/probot,bkeepers/PRobot,probot/probot,probot/probot