Move tests in flx_sync.cpp to baas/ directory

[sync-by-unito[bot]]

To support RCORE-1510, which will be needed for the network faultyness testing, migrate the tests in flx_sync.cpp (which all rely on BAAS for FLX testing) to a separate directory and break out the tests into separate files to reduce the source file size.

[sync-by-unito[bot]]

➤ michael-wb commented:

Since the Catch2 tags have proved to be a viable solution, a separate baas executable is no longer required. Taking this issue off the "Network faultyness simulated testing" project and adding to the backlog to break up the _flxsync.cpp testing file into smaller files and the baas/ directory is no longer needed.