[CI] Port in use causes panic in unit tests #3661

[701](https://github.com/gophercloud/gophercloud/actions/runs/23549763309/job/68560162724?pr=3659#step:5:2719)
=== FAIL: openstack/objectstorage/v1/objects/testing TestCreateTempURL (0.00s)
    http_responses.go:53: Failed to listen to 127.0.0.1:33200: listen tcp 127.0.0.1:33200: bind: address already in use
panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x6f8556]

goroutine 64 [running]:
testing.tRunner.func1.2({0x7a6620, 0xb2a990})
	/opt/hostedtoolcache/go/1.25.0/x64/src/testing/testing.go:1872 +0x237
testing.tRunner.func1()
	/opt/hostedtoolcache/go/1.25.0/x64/src/testing/testing.go:1875 +0x35b
panic({0x7a6620?, 0xb2a990?})
	/opt/hostedtoolcache/go/1.25.0/x64/src/runtime/panic.go:783 +0x132
net/http/httptest.(*Server).Start(0xc00024a690)
	/opt/hostedtoolcache/go/1.25.0/x64/src/net/http/httptest/server.go:132 +0x96
github.com/gophercloud/gophercloud/v2/testhelper.SetupPersistentPortHTTP(0xc00020e700, 0x81b0)
	/home/runner/work/gophercloud/gophercloud/testhelper/http_responses.go:56 +0x212
github.com/gophercloud/gophercloud/v2/openstack/objectstorage/v1/objects/testing.TestCreateTempURL(0xc00020e700)
	/home/runner/work/gophercloud/gophercloud/openstack/objectstorage/v1/objects/testing/requests_test.go:507 +0x45
testing.tRunner(0xc00020e700, 0x83a100)
	/opt/hostedtoolcache/go/1.25.0/x64/src/testing/testing.go:1934 +0xea
created by testing.(*T).Run in goroutine 1
	/opt/hostedtoolcache/go/1.25.0/x64/src/testing/testing.go:1997 +0x465
exit status 1