Add testcase.yaml

This commit adds testcase.yaml, enabling us to execute test cases
using Twister. For more information on Twister, refer to:
https://docs.zephyrproject.org/latest/develop/test/twister.html

As our current emphasis is primarily on ARM Cortex-M3, the
`integration_platforms` have been configured for targets with QEMU
featuring ARM Cortex-M3.

Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>

testcase.yaml

@@ -0,0 +1,6 @@
+tests:
+  libcsp.subsec:
+    integration_platforms:
+      - qemu_cortex_m3
+      - mps2_an385
+    tags: libcsp