Flaky test `api_branch_test`

[silverwind]

Example: https://github.com/go-gitea/gitea/actions/runs/9342097699/job/25709669145

    api_branch_test.go:257: 
        	Error Trace:	/home/runner/work/gitea/gitea/tests/integration/api_branch_test.go:257
        	Error:      	"[0xc03bbdf290 0xc03bbdf320 0xc03bbdf3b0]" should have 5 item(s), but has 3
        	Test:       	TestAPICreateBranchWithSyncBranches

[wxiaoguang]

No flaky anymore