doc: remove 'you' from writing-tests.md

Noticed this while reading through writing-tests.md today.
As per style guide avoid the use of you, your etc.
Rational as per: http://www2.ivcc.edu/rambo/tip_formal_writing_voice.htm

PR-URL: https://github.com/nodejs/node/pull/13319


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>