[teststabilization] RFR: 8229486: Replace wildcard address with loopback or local host in tests - part 21

[Daniel Fuchs]

Hi,

Please find below a fix to avoid intermittent failures
in a bunch of network tests:

8229486: Replace wildcard address with loopback or local host
          in tests - part 21
https://bugs.openjdk.java.net/browse/JDK-8229486

TcpKeepAliveTest, GetProxyPort, ImplicitFileName,
IsAvailable, IsKeepingAlive, OpenServer,
KeepAliveStreamCloseWithWrongContentLength,
   => avoid using wildcard using the usual technique

StreamingOutputStream, UserAuth, UserCookie
   => avoids wildcard  but use a slightly different technique
      to better adhere to the original test.

SetIfModifiedSince:
   => this the outlier: the server is expecting only one
      connection. This is fragile as any rogue connection from
      outside can wedge the test. The server side of the test
      is instead updated to try and recover from rogue
      connections.

best regards,

-- daniel