91226: batcheval: deflake `TestAddSSTableSSTTimestampToRequestTimestampRespectsClosedTS` r=erikgrinaker a=erikgrinaker

Flake introduced by #87264. `@tbg` Can you just quickly verify that this flake is expected given your Raft changes? Seems reasonable to me.

Resolves #91211.

Release note: None

Co-authored-by: Erik Grinaker <[email protected]>