Fix WAL compression fragmentation test (#10402)

Summary:
Previously the "Fragmentation" test didn't cover fragmentation because the WAL data was compressible into trivial size. This PR changes it to use random data so the post-compression size is large enough to require fragmentation.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10402

Reviewed By: cbi42

Differential Revision: D38065596

Pulled By: ajkr

fbshipit-source-id: 0d5f89ca14d33546501a74b5d4fafbadc28a46a7

db/log_test.cc

@@ -960,12 +960,19 @@ TEST_P(CompressionLogTest, Fragmentation) {
     return;
   }
   ASSERT_OK(SetupTestEnv());
-  Write("small");
-  Write(BigString("medium", 50000));
-  Write(BigString("large", 100000));
-  ASSERT_EQ("small", Read());
-  ASSERT_EQ(BigString("medium", 50000), Read());
-  ASSERT_EQ(BigString("large", 100000), Read());
+  Random rnd(301);
+  const std::vector<std::string> wal_entries = {
+      "small",
+      rnd.RandomBinaryString(3 * kBlockSize / 2),  // Spans into block 2
+      rnd.RandomBinaryString(3 * kBlockSize),      // Spans into block 5
+  };
+  for (const std::string& wal_entry : wal_entries) {
+    Write(wal_entry);
+  }
+
+  for (const std::string& wal_entry : wal_entries) {
+    ASSERT_EQ(wal_entry, Read());
+  }
   ASSERT_EQ("EOF", Read());
 }