Move the Chromium Test Executable API spec into markdown.

This CL moves the bit.ly/chromium-test-runner-api spec from
https://docs.google.com/document/d/1i9kC7xrq0Slfcx8nTjoKT1TQ4_sm3Vx2XpeIVFIidHA/edit
(a Google Docs document) into a (Markdown) file checked into
the repo at //docs/testing/test_executable_api.md.

Change-Id: I1a231afa18bace25e40445673f5b2b997d4427cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278179
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784948}