TL;DR
Use path.join(__dirname, '..', 'test', 'karma.conf.js'). Prevent use of slashes.
Long Answer
As a lot of answers have pointed out, using path module is probably the best way.
However, most of the solutions here have gone back to using slashes like:
path.join(__dirname+'../test/karma.conf.js')
However, by doing this, you’re beating the purpose of using path. One uses path to do operations irrespective of the underlying OS (Linux, Windows etc). Just to give a bit of insight, you can do the path operations directly as string operations (like __dirname + '../test/karma.conf.js'. You do not do this because Linux uses forward slashes ( / ), Windows uses backward slashes ( \ ). This makes your application prone to errors when you port it across operating systems.
Thus, the better way would be:
path.join(__dirname, '..', 'test', 'karma.conf.js')
And of course, coming back – prevent use of slashes in your path.join, instead spread out your params.