availablecoins_tests fails intermittently with "invalid schnorr signature"

[delta1]

upstream pull bitcoin/bitcoin#24584 introduces a new unit test availablecoins_tests

about 30% of the time the test fails with "invalid schnorr signature" when trying to verify the signature of the transaction it has just created

this only occurs when the variants are all run in the same test.

splitting the tests into individual boost test cases works around the issue

this requires further investigation

debugging commit: delta1@0739a57