including externs definitions in "codeql test"

Would it be possible to give the “codeql test” command an option to include externs and/or make this something that one can specify in a config file? Right now, the approach we have taken is to copy/symlink externs definitions into the directory containing test files, but this solution seems unsatisfactory/fragile, particularly if one wants to use the standard externs definitions.

My question relates to JavaScript, but the same issue presumably arises for other languages.