Unit tests can fail because of async operations

Overview:

ApiTestBase$verifyPostResponse has been using verifyInvoiceSummaryUpdateEvent to wait for some update events to finish. As this is no longer done async, verifyInvoiceSummaryUpdateEvent is not longer needed. But without a new sleep to replace it, tests can fail because some async operations happen at the same time as the next tests (verifyInvoiceSummaryUpdateEvent had a minimum 50ms delay which was helping to avoid issues).
The sleep workaround introduced in the resolution for MODINVOICE-233 is not ideal because the right value depends on the environment, and it slows down the tests.

Steps to reproduce:
(errors may vary depending on the running environment)

• Remove the sleep in ApiTestBase$verifyPostResponse

• mvn test
  [...]
  16:28:08 [] [] [] [] INFO BatchVoucherExportsApiTest === Test Put Batch voucher export === 16:28:08 [] [] [] [] INFO ApiTestBase Using mock datafile: mockdata/batchVoucherExports/566c9156-e52f-4597-9fee-5ddac91d14f2.json 16:28:08 [] [] [] [] INFO MockServer id: null 16:28:08 [] [] [] [] INFO ApiTestBase Using mock datafile: mockdata/batchVouchers/null.json 16:28:08 [] [] [] [] ERROR HelperUtils Exception calling GET /batch-voucher-storage/batch-vouchers/null?lang=en java.util.concurrent.CompletionException: org.folio.invoices.rest.exceptions.HttpException: null
  [...]
  [INFO] Results:
  [INFO] 
  [ERROR] Failures: 
  [ERROR] ApiTestSuite$BatchVoucherExportsApiTestNested>BatchVoucherExportsApiTest.testUpdateBatchVoucherExport:130 
  Expected: is "Updated message"
   but: was "Batch voucher was not found"
  [INFO] 
  [ERROR] Tests run: 384, Failures: 1, Errors: 0, Skipped: 4

Resolution

• A better workaround to wait for async operations to finish without using a fixed sleep value, OR
• Change the tests to support parallel execution (this could be complicated and probably require changes in MockServer)