fix: improve functional test stability and CI reliability

- Fix parallel test execution conflicts by moving test files to build dir
- Improve error handling in ci-build.sh with proper exit codes
- Update test file paths for out-of-tree build compatibility
- Add test-files and test-logs to .gitignore
- Add clean-functional-tests target to Makefile.am

tests/functionaltests/proftest.c

@@ -137,7 +137,7 @@ void
 _cleanup_dirs(void)
 {
     char cmd[512];
-    snprintf(cmd, sizeof(cmd), "rm -rf ./tests/functionaltests/files/%d", stub_port);
+    snprintf(cmd, sizeof(cmd), "rm -rf ./test-files/%d", stub_port);
     int res = system(cmd);
     if (res == -1) {
         assert_true(FALSE);
@@ -241,10 +241,11 @@ init_prof_test(void **state)
     }
 
     // Generate unique XDG paths based on stub_port for parallel execution
+    // Use ./test-files/ in current (build) directory for out-of-tree builds compatibility
     snprintf(xdg_config_home, sizeof(xdg_config_home),
-             "./tests/functionaltests/files/%d/xdg_config_home", stub_port);
+             "./test-files/%d/xdg_config_home", stub_port);
     snprintf(xdg_data_home, sizeof(xdg_data_home),
-             "./tests/functionaltests/files/%d/xdg_data_home", stub_port);
+             "./test-files/%d/xdg_data_home", stub_port);
 
     // Give stabber server thread time to start listening
     usleep(100000); // 100ms