feat(test): make unittest pattern configurable and pass TEST_PATTERN into containers

This update introduces a configurable TEST_PATTERN variable in the Makefile,
allowing selective execution of unit, integration, and E2E tests without
modifying scripts.

Key changes:
- Add TEST_PATTERN (default: test_*.py) to Makefile and export it.
- Inject TEST_PATTERN into all test containers via `-e TEST_PATTERN=...`.
- Update test-unit.sh, test-integration.sh, and test-e2e.sh to use
  `-p "$TEST_PATTERN"` instead of a hardcoded pattern.
- Ensure flexible test selection via:
      make test-e2e TEST_PATTERN=test_install_pkgmgr_shallow.py

This enables fast debugging, selective test runs, and better developer
experience while keeping full compatibility with CI defaults.

https://chatgpt.com/share/69385400-2f14-800f-b093-bb03c8ef9c7f

Makefile

@@ -12,7 +12,7 @@ NIX_CACHE_VOLUME := pkgmgr_nix_cache
 # Distro list and base images
 # (kept for documentation/reference; actual build logic is in scripts/build)
 # ------------------------------------------------------------
-DISTROS := arch debian ubuntu fedora centos
+DISTROS 		  := arch debian ubuntu fedora centos
 BASE_IMAGE_ARCH   := archlinux:latest
 BASE_IMAGE_DEBIAN := debian:stable-slim
 BASE_IMAGE_UBUNTU := ubuntu:latest
@@ -27,6 +27,10 @@ export BASE_IMAGE_UBUNTU
 export BASE_IMAGE_FEDORA
 export BASE_IMAGE_CENTOS
 
+# PYthon Unittest Pattern
+TEST_PATTERN	:= test_*.py
+export TEST_PATTERN
+
 # ------------------------------------------------------------
 # PKGMGR setup (developer wrapper -> scripts/installation/main.sh)
 # ------------------------------------------------------------