Make sure that the directory we're about to save a JSON DB blob into actually exists.
Expose the mechanism for determining the JSON database file locations.
Put the tests into the temp folder.
Merge from develop
 ChangeLog                         |   6 --
 Cpp/fost-jsondb-test/backup.cpp   | 150 +++++++++++++++++++-------------------
 Cpp/fost-jsondb-test/filetest.cpp | 119 +++++++++++++++---------------
 Cpp/fost-jsondb/blobdb.cpp        |  23 +++---
 Cpp/include/fost/jsondb.hpp       |   5 +-
 5 files changed, 146 insertions(+), 157 deletions(-)