[now-static-build] Exit dev server child processes upon SIGINT/SIGTERM (#3136)
[now-static-build] Exit dev server child processes upon SIGINT/SIGTERM (#3136)

Explicitly send the SIGINT / SIGTERM signal to `now dev` server child processes, so that they are not left running when running the now-dev unit tests.

Related to #3113 which has hanging unit tests that never "complete".
zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli