:construction_worker: added task to check amalgamation #906
:construction_worker: added task to check amalgamation #906

- Builds and runs test cases from individual sources rather than from amalgamation.
- Checks whether amalgamation would create the same single-header file as checked in.

nlohmann/json,nlohmann/json,theodelrieu/json,nlohmann/json,theodelrieu/json