Deduplicate code in URLConnectionTest through a helper method.

This is in preparation for a follow-up CL to modify the shared
code.

Test: cts-tradefed run cts -m CtsLibcoreTestCases \
        -t libcore.java.net.URLConnectionTest

Change-Id: I89cce7ed18491f343fa4ee9d0ad721d0e7520541