We should create an incremental backup related test that, essentially, reproduces WT-6136. The characteristics of this test would be:

• Populate a very large table that requires expansion and reallocation of the bitmap. Therefore, set the granularity to its smallest allowed configuration size of 1Mb and generate a file that is larger than 256Mb.
• Create a workload that repeatedly modifies a small portion of the table. Perhaps repeatedly modifies even a single key only.
• Perform full and then incremental backups of this file, making multiple copies.
• On the incremental, for each block returned during the backup, verify that the block is different than the content of the block from the last (full or incremental) backup version of the block.