v8.takeCoverage() method allows the user to write the coverage started by
NODE_V8_COVERAGE to disk on demand. This method can be invoked multiple
times during the lifetime of the process, each time the execution counter will
be reset and a new coverage report will be written to the directory specified
When the process is about to exit, one last coverage will still be written to
v8.stopCoverage() is invoked before the process exits.