db_stress print TestMultiGet error value in hex (#10753)

Summary:
Without this fix, db_crashtest.py could fail with useless output such as: `UnicodeDecodeError: 'utf-8' codec can't decode byte 0xa0 in position 267: invalid start byte`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10753

Reviewed By: hx235

Differential Revision: D39905809

Pulled By: ajkr

fbshipit-source-id: 50ba2cf20d206eeb168309cec137e827a34c8f0b

db_stress_tool/no_batched_ops_stress.cc

@@ -614,7 +614,8 @@ class NonBatchedOpsStressTest : public StressTest {
                   keys[i].ToString(true).c_str());
           fprintf(stderr, "MultiGet returned value %s\n",
                   values[i].ToString(true).c_str());
-          fprintf(stderr, "Get returned value %s\n", value.c_str());
+          fprintf(stderr, "Get returned value %s\n",
+                  Slice(value).ToString(true /* hex */).c_str());
           is_consistent = false;
         }
       }