Ismo, I think you have the most context on this test - would you be able to take a look?

I can repro locally with
bazel test //test/extensions/transport_sockets/tls:ssl_socket_test --runs_per_test=1000
but if you have trouble the extra flags in test/integration/README.md deflaking section might help.

Sample failure:
https://circleci.com/gh/envoyproxy/envoy/276851?utm_campaign=workflow-failed&utm_medium=email&utm_source=notification

[ RUN ] IpVersions/SslSocketTest.RsaPrivateKeyProviderSyncSignFailure/IPv6
unknown file: Failure

Unexpected mock function call - returning directly.
Function call: onEvent(4-byte object <02-00 00-00>)
Google Mock tried the following 1 expectation, but it didn't match:

test/extensions/transport_sockets/tls/ssl_socket_test.cc:416: EXPECT_CALL(server_connection_callbacks, onEvent(options.expectedServerCloseEvent()))...
Expected arg #0: is equal to 4-byte object <00-00 00-00>
Actual: 4-byte object <02-00 00-00>
Expected: to be called once
Actual: never called - unsatisfied and active
Stack trace:
0x1c62f29: testing::internal::GoogleTestFailureReporter::ReportFailure()
0x4c85c5: testing::internal::Expect()
0x1c770a5: testing::internal::UntypedFunctionMockerBase::UntypedInvokeWith()
0x4ec4d0: testing::internal::FunctionMocker<>::Invoke()
0x4ec3e3: Envoy::Network::MockConnectionCallbacks::onEvent()
0x127a906: Envoy::Network::ConnectionImpl::raiseEvent()
0x56c636: Envoy::Extensions::TransportSockets::Tls::SslSocket::doHandshake()
0x56d202: Envoy::Extensions::TransportSockets::Tls::SslSocket::doWrite()
0x127ade9: Envoy::Network::ConnectionImpl::onWriteReady()
0x127c879: Envoy::Network::ConnectionImpl::onFileEvent()
0x127ec7e: Envoy::Network::ConnectionImpl::ConnectionImpl()::$_2::operator()()
0x127eb31: std::_Function_handler<>::_M_invoke()
0x9dbbfa: std::function<>::operator()()
0x1271e47: Envoy::Event::FileEventImpl::assignEvents()::$_0::operator()()
0x1271c78: Envoy::Event::FileEventImpl::assignEvents()::$_0::__invoke()
0x1b947eb: event_process_active_single_queue
0x1b912b0: event_base_loop
0x13085bc: Envoy::Event::LibeventScheduler::run()
0x126bfa8: Envoy::Event::DispatcherImpl::run()
0x43b47f: Envoy::Extensions::TransportSockets::Tls::(anonymous namespace)::testUtil()
0x467adb: Envoy::Extensions::TransportSockets::Tls::SslSocketTest_RsaPrivateKeyProviderSyncSignFailure_Test::TestBody()
0x1c5b51e: testing::internal::HandleSehExceptionsInMethodIfSupported<>()
0x1c4b1ae: testing::internal::HandleExceptionsInMethodIfSupported<>()
0x1c35023: testing::Test::Run()
0x1c35c35: testing::TestInfo::Run()
... Google Test internal frames ...

unknown file: Failure

Unexpected mock function call - returning directly.
Function call: onEvent(4-byte object <02-00 00-00>)
Google Mock tried the following 1 expectation, but it didn't match:

test/extensions/transport_sockets/tls/ssl_socket_test.cc:414: EXPECT_CALL(client_connection_callbacks, onEvent(Network::ConnectionEvent::RemoteClose))...
Expected arg #0: is equal to 4-byte object <00-00 00-00>
Actual: 4-byte object <02-00 00-00>
Expected: to be called once
Actual: never called - unsatisfied and active
Stack trace:
0x1c62f29: testing::internal::GoogleTestFailureReporter::ReportFailure()
0x4c85c5: testing::internal::Expect()
0x1c770a5: testing::internal::UntypedFunctionMockerBase::UntypedInvokeWith()
0x4ec4d0: testing::internal::FunctionMocker<>::Invoke()
0x4ec3e3: Envoy::Network::MockConnectionCallbacks::onEvent()
0x127a906: Envoy::Network::ConnectionImpl::raiseEvent()
0x56c636: Envoy::Extensions::TransportSockets::Tls::SslSocket::doHandshake()
0x56d202: Envoy::Extensions::TransportSockets::Tls::SslSocket::doWrite()
0x127ade9: Envoy::Network::ConnectionImpl::onWriteReady()
0x127c879: Envoy::Network::ConnectionImpl::onFileEvent()
0x127ec7e: Envoy::Network::ConnectionImpl::ConnectionImpl()::$_2::operator()()
0x127eb31: std::_Function_handler<>::_M_invoke()
0x9dbbfa: std::function<>::operator()()
0x1271e47: Envoy::Event::FileEventImpl::assignEvents()::$_0::operator()()
0x1271c78: Envoy::Event::FileEventImpl::assignEvents()::$_0::__invoke()
0x1b947eb: event_process_active_single_queue
0x1b912b0: event_base_loop
0x13085bc: Envoy::Event::LibeventScheduler::run()
0x126bfa8: Envoy::Event::DispatcherImpl::run()
0x43b47f: Envoy::Extensions::TransportSockets::Tls::(anonymous namespace)::testUtil()
0x467adb: Envoy::Extensions::TransportSockets::Tls::SslSocketTest_RsaPrivateKeyProviderSyncSignFailure_Test::TestBody()
0x1c5b51e: testing::internal::HandleSehExceptionsInMethodIfSupported<>()
0x1c4b1ae: testing::internal::HandleExceptionsInMethodIfSupported<>()
0x1c35023: testing::Test::Run()
0x1c35c35: testing::TestInfo::Run()
... Google Test internal frames ...

Terminated
-- Test timed out at 2019-09-16 13:29:33 UTC --