StaticFileMiddlewareTests should use dynamic ports

[mikeharder]

StaticFileMiddlewareTests should use dynamic ports rather than hardcoding port 12345. For ServerType.Kestrel, the app should bind to dynamic port "0" and query the app to find the assigned port, since this is 100% reliable. For ServerType.WebListener, the app should use GetNextPort() which is less reliable than directly binding to "0", but should be more reliable than a hardcoded port.

https://github.com/aspnet/StaticFiles/blob/5d2b1000f19dc3e892611f387b46cd519976a9f0/test/Microsoft.AspNetCore.StaticFiles.FunctionalTests/StaticFileMiddlewareTests.cs#L181