Convert VpnTest to JUnit4

This is to fix the error in errorprone target, the error message
is: [JUnit4ClassUsedInJUnit3] Assume cannot be used inside a
JUnit3 class. Convert your class to JUnit4 style.
The error method is: assumeTrue()

Bug: 191918368
Test: atest CtsHostsideNetworkTests:HostsideVpnTests
Change-Id: Ieb606d2ba31cc19510425114c76fd77417227f2c
1 file changed