mangrove-solidity: Fix and reenable flaky test in test-defensive

[dontrolle]

Describe the bug
After a number of failing CI-runs, it was determined that the .once() event-listener in test-defensive.js would sometime fail to fire, leading the test to stall indefinitely.

To Reproduce
Uncomment the commented test-case ""Price fed strat" in packages/mangrove-solidity/test/test-defensive.js and run the tests in CI (maybe several times) - local runs have typically failed to repro the problem.

Expected behavior
The test never stalls.

Several lines of attack are possible here:

• Investigate why the event-listener doesn't fire sometimes
• Ensure that the test times out, even if the event is not received.
• Use a different pattern / rewrite the test.

[jkrivine]

this test is no longer part of the test suite.