[5.0] improve reliability of http_plugin_unit_tests via port workaround

[spoonincode]

For unknown reasons, sometimes when http_plugin_test_fixture is destroyed http_plugin's listeners are not destroyed despite http_plugin itself being destroyed. When this occurs the port that listener was listening on gets tied up for the remainder of http_plugin_unit_tests causing failures for the remaining test cases.

It's not clear to me why this is occurring. I suspect either some static somewhere or something I don't expect holding listener's shared_ptr.

In the mean time just don't reuse the same port numbers among all the different test cases. It is rather curious that the valid_category_addresses test case already avoided 8888 that the prior http_plugin_unit_tests test case used.

[ericpassmore]

Note:start
group: STABILITY
category: TEST
summary: Improve http plugin unit tests by avoiding dangling listeners.
Note: end