jest should not use forceExit but needs to call jest through node API

@kenotron 2019-09-10 16:41:45发表于 microsoft/just

Currently, we trigger jest with a CLI interface. This is okay, but in order to have the CLI quite FOR SURE when all tests are over is through --forceExit. This unfortunately generates an ugly output, we need to either somehow filter that output or have jestTask call jest node api to trigger the runner.