Address EMFILE (too many open files) errors for clean `grunt test` runs

[benjamn]

cc @jeffmo

[benjamn]

Recommended workaround for the time being: just run grunt test again; due to caching, it will pick up where it left off.

[benjamn]

Confirming that reducing MAX_READ_COUNT for Commoner's file reading queue makes the problem go away:
https://github.com/benjamn/commoner/blob/f006bac9c9152e6ffcb33eb8cc3a6f994f57a794/lib/util.js#L56