[Fuchsia] Improve the Fuchsia documentation.

* Move instructions about running tests from //fuchsia/README.md to
  //docs/fuchsia_build_instructions.md.
* Rewrite the "Test code" section in //fuchsia/README.md to include
  test guidelines and update the test code structure.

Bug: 978177
Change-Id: I43802de20a1fcb45fced1ea5ffa77b60a8566c59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1682463
Reviewed-by: Sergey Ulanov <[email protected]>
Commit-Queue: Fabrice de Gans-Riberi <[email protected]>
Cr-Commit-Position: refs/heads/master@{#674138}
2 files changed