CONTRIBUTING.md: Add unit testing guidelines
CONTRIBUTING.md: Add unit testing guidelines

See rationale in the bug.

Bug: b:243053027
Change-Id: I5315781e360f6f2feea2adc58ec02bd11c7a7c70
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3898110
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Commit-Queue: Clarissa Garvey <18c93150ce036b251830022c755392b5185f2959@chromium.org>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Reviewed-by: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm