Moved folder function to inner level

The function is implementation detail, and should not be publically accessible.
Renamed MetaData to TestDataMap

Originally, the module was created for storing meta data, but the same
module and types are also used to store context data. Therefore
TestDataMap seems like a better name.
Extracted get and merge helper function in tests

To reduce the number of places to change, if the TestDataMap is renamed
again.
Experimental alternate syntax for metadata