Test that the system doesn't accept non-/96 NAT64 prefixes.

Other system components (clatd, netd's DNS64 synthesis) don't
support non-/96 prefixes, so such prefixes must be ignored.
CTS already checks that LinkProperties#setNat64Prefix throws
IllegalArgumentException when passed an invalid prefix length.
Here we test that nothing happens (e.g., the network stack
doesn't crash) when that exception is thrown.

Bug: 153694684
Test: new unit tests
Change-Id: I8a57e171d472532b930c8d5c33e11a99024cf128