feat(test): add comprehensive test infrastructure

- Add test target to both bmake.mk and gmake.mk Makefiles
- Add test-related artifacts to .gitignore (excluding DLLs/shared libs)
- Create test directory with cross-platform Makefiles (bmake.mk, gmake.mk)
- Implement cross-platform test program with signal handling (test.c)
- Add test module source for compilation (test_mod.c)
- Set up automated test environment with mods/ directory
- Support both POSIX and Windows platforms

The test infrastructure allows running integration tests via 'make test'
and demonstrates stk library functionality with dynamic module loading.
Generated DLLs/shared libraries are excluded from version control.

test/test_mod.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+
+int stk_mod_init(void)
+{
+	printf("test mod initialized!\n");
+	return 0;
+}
+
+void stk_mod_shutdown(void) { printf("test mod shut down.\n"); }