Investigate CI failures when dummy app's denali-build.js is not present

[davewasmer]

So this is a bizarre one:

Normally, a denali-build.js file is not required anywhere, as long as you are okay with the default build process. However, it seems that addons (including denali-babel itself) that use denali-babel, and don't have a test/dummy/denali-build.js, have their build commands hang in CI environments. But only if that build command is issued from a CommandAcceptanceTest (i.e. denali-eslint has a command acceptance test that tests the linter output, which tries to run build, and hangs).

After inspecting the hanging process, it seems like the broccoli-babel-transpiler has worker processes that are keeping the parent process alive with their IPC channels.

Not sure why adding test/dummy/denali-build.js fixes the issue, or why it only happens on CI, or why it only happens for commands shelled out from CommandAcceptanceTest (as opposed to when invoked directly in a terminal).