next-checks: add cargo test checks

Currently, we are not running unit tests on trusted-firmware.

 * Add a script to run `cargo test`.

`cargo test` does not require building fip or any other component
and is platform independent, so the test is added to a script
instead of using the `groups`/`run-config` framework.

Change-Id: I7ffb7bb9f8581f588fb62a96b3d234d576b33f3f
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
1 file changed