5824: Simplify keepproxy test setup.

5824: Simplify keepproxy test setup.

For testing, configure KeepClient to use the test proxy server,
instead of telling the API about the test proxy server and using env
vars to trick the client into discovering it.

Let the OS choose an available port for each test server instead of
pre-assigning an arbitrary port number to each test case.

Use a properly formatted fake UUID.