test: sandbox: Move sandbox test docs into doc/develop

At present some of the documentation about running sandbox tests is in the
sandbox docs. It makes more sense to put it in with the other testing
docs, with a link there from sandbox. Update the documentation
accordingly.

Also add a paragraph explaining why sandbox exists and the test philosophy
that it uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
2 files changed