Test flakiness due to hard-coded ports

[sporksmith]

Some tests open a listening socket on a hard-coded port number. While these are fine when running under Shadow, when we run them on the native host those ports may not be available, causing the test to fail.

[sporksmith]

Pull request #709 fixes some, but not all, of these.

[sporksmith]

#763 (particularly e4900a5) demonstrates a way of doing this for multi-process tests. Outside of Shadow, we can use a fifo to pass the port number from the server process to the client process.

Unfortunately fifo's don't currently work properly under Shadow, but it's OK to use hard-coded ports under Shadow, since they should be reliably available there. i.e. the tests can take a command-line parameter either specifying a hard-coded port (for use when running under Shadow), or a path to a fifo to use to communicate the port.

[sporksmith]

I think #787 took care of the rest of these.