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

Makefile.am

@@ -313,7 +313,7 @@ FUNC_TEST_GROUPS = 1 2 3 4
 
 check-functional-parallel: tests/functionaltests/functionaltests
 	@echo "Running functional tests in parallel ($(words $(FUNC_TEST_GROUPS)) groups)..."
-	@mkdir -p $(builddir)/test-logs
+	@mkdir -p $(builddir)/test-logs $(builddir)/test-files
 	@pids=""; \
 	for g in $(FUNC_TEST_GROUPS); do \
 		./tests/functionaltests/functionaltests $$g > $(builddir)/test-logs/group$$g.log 2>&1 & \
@@ -372,7 +372,10 @@ clean-local:
 	rm -f $(git_include) $(git_include).in
 endif
 
-.PHONY: my-prof.supp
+clean-functional-tests:
+	rm -rf $(builddir)/test-files $(builddir)/test-logs
+
+.PHONY: my-prof.supp clean-functional-tests
 my-prof.supp:
 	@sed '/^# AUTO-GENERATED START/q' prof.supp > $@
 	@printf "\n\n# glib\n" >> $@