Migrate HostsideNetworkTestCase to use JUnit4

This is no-op refactoring but allows subsequent
changes to use modern JUnit4 functionalities such
as @BeforeClass annotation.

Test: atest HostsideNetworkCallbackTests \
      HostsideConnOnActivityStartTest \
      HostsideSelfDeclaredNetworkCapabilitiesCheckTest \
      HostsideVpnTests HostsideNetworkPolicyManagerTests
Bug: 289002798
Change-Id: I503c13ffcb239c4535781ba5d660a3280b7a395b