test: Add a simple script to run tests on sandbox

test: Add a simple script to run tests on sandbox

A common check before sending patches is to run all available tests on
sandbox. But everytime I do this I have to look up the README. This presents
quite a barrier to actually doing this.

Add a shell script to help. To run the tests, type:

   test/run

in the U-Boot directory, which should be easy to remember.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Teddy Reed <teddy.reed@gmail.com>