Windows doesn't properly support SO_REUSEADDR, so that's disabled in Python, and thus rebinding fails. For successful testing, reuse port instead.

sanic/testing.py

@@ -95,7 +95,10 @@ class SanicTestClient:
 
         if self.port:
             server_kwargs = dict(
-                host=host or self.host, port=self.port, **server_kwargs
+                host=host or self.host,
+                port=self.port,
+                reuse_port=True,  # Try to avoid test failures on Windows
+                **server_kwargs,
             )
             host, port = host or self.host, self.port
         else: