Use data generation for synthetic source unit tests

[lkts]

Similar to #115257 (see BlockLoaderTestCase) we can rework synthetic source tests for mappers based on MapperTestCase#syntheticSourceSupport to use data generation. This will increase the coverage of such cases because data generation covers more mapping parameters and inputs.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)