topotato: add test for bgp vrf leak

[eznix86]

The test passes, but the existing code has memory leak detection:

def test_memory_leak():
    "Run the memory leak test and report results."
    tgen = get_topogen()
    if not tgen.is_memleak_enabled():
        pytest.skip("Memory leak test/report is disabled")

    tgen.report_memory_leaks()

Should it be added ?