[now-static-build] Exit dev server child processes upon SIGINT/SIGTERM

[TooTallNate]

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".

[codecov-io]

Codecov Report

Merging #3136 into canary will decrease coverage by 2.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           canary    #3136      +/-   ##
==========================================
- Coverage   13.29%   11.24%   -2.06%     
==========================================
  Files         268      268              
  Lines       10332    10071     -261     
  Branches     1218     1315      +97     
==========================================
- Hits         1374     1132     -242     
+ Misses       8895     8846      -49     
- Partials       63       93      +30

Impacted Files	Coverage Δ
src/util/output/create-output.ts	31.25% <0%> (-12.34%)	⬇️
src/util/prefer-v2-deployment.ts	56.25% <0%> (-5.66%)	⬇️
src/util/dev/yarn-installer.ts	85.71% <0%> (-5.03%)	⬇️
src/util/dev/server.ts	58.97% <0%> (-4.88%)	⬇️
src/util/dev/builder.ts	72.51% <0%> (-4.41%)	⬇️
src/util/dev/router.ts	78.57% <0%> (-4.13%)	⬇️
src/util/get-files.ts	87.77% <0%> (-3.89%)	⬇️
src/util/projects/get-project-by-id-or-name.ts	25% <0%> (-3.58%)	⬇️
src/util/dev/builder-cache.ts	77.27% <0%> (-3.48%)	⬇️
src/util/metrics.ts	70.58% <0%> (+2.4%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d38e464...4ffece8. Read the comment docs.