logv2_test --verbose=v --suite=LogV2Test

{"t":{"$date":"2020-04-17T02:19:51.470Z"},"s":"I",  "c":"TEST",     "id":4680100, "ctx":"main","msg":"FAIL","attr":{"test":"UserAssert","type":"TestAssertionFailureException","error":"Expected lines.back() == ex.reason() (uasserting logUser assertion == uasserting log) @src/mongo/logv2/logv2_test.cpp:1600","extra":"........"}}

 The last log line we get here unexpectedly has the LOGV2_DEBUG Debug(1) severity message: "User assertion" tacked onto it, which comes from inside the body of uassertedWithLocation in util/assert_util.cpp.

Running without the verbose flag there is no failure.
SO maybe the test should temporarily stop debug instrumentation messages from being emitted while the test runs.