URLConnectionTest: Convert to JUnit 4.

This is preparation for adoption of a @CoreCompatChangeRule, which
requires JUnit 4.

Test: Checked that the following runs the same number (152) of test
      cases both before and after this CL:
      atest CtsLibcoreTestCases:libcore.java.net.URLConnectionTest

Change-Id: I0512d5a617038e3e75753f9af8d86239551eeef1
1 file changed