Use unix_sockname fixture for test_run_app_preexisting_unix_socket (#11498)

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## What do these changes do?

Fixes the tests in sandboxed build environments where `/tmp` is not
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Author: emilazy

tests/test_run_app.py

@@ -646,23 +646,22 @@ def test_run_app_preexisting_inet6_socket(
 
 @skip_if_no_unix_socks
 def test_run_app_preexisting_unix_socket(
-    patched_loop: asyncio.AbstractEventLoop, mocker: MockerFixture
+    patched_loop: asyncio.AbstractEventLoop, unix_sockname: str, mocker: MockerFixture
 ) -> None:
     app = web.Application()
 
-    sock_path = "/tmp/test_preexisting_sock1"
     sock = socket.socket(socket.AF_UNIX)
     with contextlib.closing(sock):
-        sock.bind(sock_path)
-        os.unlink(sock_path)
+        sock.bind(unix_sockname)
+        os.unlink(unix_sockname)
 
         printer = mock.Mock(wraps=stopper(patched_loop))
         web.run_app(app, sock=sock, print=printer, loop=patched_loop)
 
         patched_loop.create_server.assert_called_with(  # type: ignore[attr-defined]
             mock.ANY, sock=sock, backlog=128, ssl=None
         )
-        assert f"http://unix:{sock_path}:" in printer.call_args[0][0]
+        assert f"http://unix:{unix_sockname}:" in printer.call_args[0][0]
 
 
 def test_run_app_multiple_preexisting_sockets(