Optionally extract build artifacts from an archive file instead of
building them.  This completes the separation between building the tests
and running them.  To use this, do something like:

    go run test-compile.go

Arrange to have the mcuboot dir and the test-images.zip on the test
target, and then run:

    go run run-tests.go -prebuilt test-images.zip

Signed-off-by: David Brown <david.brown@linaro.org>