Fix nfsroot-simple-test test reporting

This test used a simple echo pass. The qa report was not able to
understand that the test passed successfully because it eliminated
tests results of the "lava" test group.
Use the specific lava tool for reporting instead

Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>

test-cases/nfsroot-simple-boot.yaml

@@ -16,7 +16,7 @@ metadata:
 
 run:
   steps:
-    - echo pass
+    - lava-test-case nfsroot-simple-boot --result pass
 
 parse:
   pattern: 'TEST_RESULT:(?P<result>\w+):(?P<test_case_id>[^:]+):'