sanity-check-manual: add clarification about steps and pre-conditions

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

test-cases/sanity-check-manual.yaml

@@ -36,6 +36,10 @@ metadata:
        from a second machine."
     - "Once the apertis-tests folder is available in the Apertis image, follow the
        execution steps."
+    - "Make sure that no USB memory stick is plugged into the target before
+       starting the test, it will otherwise fail. If you are booting from an
+       USB stick, an error `Failed to start usbmount@dev-sda1.service` will
+       appear which is expected."
 
   expected:
     - "The command should report no failure and its output should be something
@@ -70,9 +74,9 @@ install:
 run:
   steps:
     - "# Enter test directory:"
-    - cd sanity
+    - $ cd sanity
     - "# Execute the following command:"
-    - ./sanity-check
+    - $ sudo ./sanity-check
 
 parse:
   pattern: 'TEST_RESULT:(?P<result>\w+):(?P<test_case_id>[^:]+):'