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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc