Use dynamic distro selection for UNIT and INTEGRATION tests

- Pass `distro=arch` from Makefile into test scripts
- Replace hardcoded "arch" references with "${distro}"
- Update test-unit.sh and test-integration.sh to use dynamic image names
- Improve log output to reflect selected distro

https://chatgpt.com/share/6939c98a-d428-800f-8bb8-cf72e80ba80c

Makefile

@@ -45,10 +45,10 @@ build:
 # ------------------------------------------------------------
 
 test-unit: build-missing
-	@bash scripts/test/test-unit.sh
+	@distro=arch scripts/test/test-unit.sh
 
 test-integration: build-missing
-	@bash scripts/test/test-integration.sh
+	@distro=arch scripts/test/test-integration.sh
 
 test-e2e: build-missing
 	@bash scripts/test/test-e2e.sh