lib: Fix running twister

Use integration_platforms and invoke twister properly.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

.github/workflows/build.yml

@@ -25,10 +25,10 @@ jobs:
         run: |
           west build -b custom_plank app
 
-      - name: Twister Unit Tests
+      - name: Twister Tests
         working-directory: example-application
         run: |
-          ../zephyr/scripts/twister -p qemu_cortex_m0 --testcase-root ./tests/
+          ../zephyr/scripts/twister -G --board-root boards/ --testsuite-root ./tests/
 
       - name: Archive firmware
         uses: actions/upload-artifact@v2

tests/lib/custom_lib/testcase.yaml

@@ -1,5 +1,8 @@
 common:
   tags: extensibility
+  integration_platforms:
+    - custom_plank
+    - qemu_cortex_m0
 tests:
   lib.custom_lib:
     build_only: false   # something benign to make a non-empty list