Add a test-case for nfsroot simple boot

After booting an nfsroot, an image is flashed and we only want to
ensure the image boots properly to a shell.
This test uses the nfsroot image type.

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

test-cases/nfsroot-simple-boot.yaml

+metadata:
+  name: nfsroot-simple-boot
+  format: "Apertis Test Definition 1.0"
+  image-types:
+    nfsroot: [ armhf, arm64, amd64 ]
+  image-deployment:
+    - APT
+    - OSTree
+  type: functional
+  exec-type: automated
+  priority: high
+  maintainer: "Apertis Project"
+  description: "Check that the image booted."
+
+  expected:
+    - "The output should show pass."
+
+run:
+  steps:
+    - echo pass
+
+parse:
+  pattern: 'TEST_RESULT:(?P<result>\w+):(?P<test_case_id>[^:]+):'