Temporary fix for #10678
zapwallettxes.py doesn't test anything due to a syntax error.
Fixing the syntax error causes the test to fail because of bad interaction between zapwallettxes and persistent mempool. There is a fix for that in #10330
Until #10330 is merged we should disable this test. There's no point in running a test that doesn't actually test anything, and which occasionally causes spurious Travis failures.