Flink Go XVR tests fail on TestXLang_...: Insufficient number of network buffers

When running the cross-language test suites () Flink fails on TestXLang_Multi with the following error:
```

19:29:14 2021/08/27 02:29:14 (): java.io.IOException: Insufficient number of network buffers: required
17, but only 16 available. The total number of network buffers is currently set to 2048 of 32768 bytes
each. You can increase this number by setting the configuration keys 'taskmanager.memory.network.fraction',
'taskmanager.memory.network.min', and 'taskmanager.memory.network.max'.
19:29:14 2021/08/27 02:29:14
Job state: FAILED
19:29:14 --- FAIL: TestXLang_Multi (6.26s)
```

This doesn't seem to be a parallelism problem (go test is run with "-p 1" as expected) and is only happening on this specific test.

Imported from Jira [BEAM-12815](https://issues.apache.org/jira/browse/BEAM-12815). Original Jira may contain additional context.
Reported by: danoliveira.

Start by reproducing the cross-language test suites, focusing on TestXLang_Multi and its network-buffer configuration. Inspect the test setup and Flink settings that determine available network buffers; done means the test completes without the insufficient-network-buffers failure.

Written by the indexing model from the issue text.