Test the generated code, do not write yet another marshaling routine in
tests.
Before:
```
ok git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/util/proto 0.003s coverage: 55.6% of statements
```
After:
```
ok git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/util/proto 0.003s coverage: 80.0% of statements
```
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
