tests: Mark the tests that don't need to downoad test files with the …

[LocutusOfBorg]

This way in Debian and Ubuntu we can easily skip them when no internet is available

[coveralls]

coverage: 99.186%. remained the same
when pulling eedb7a8 on LocutusOfBorg:offline-tests
into 8215dba on nlohmann:develop.

[gregmarr]

Is it better to mark the ones that don't need to download things, or the ones that do?

[nlohmann]

From the README:

Note that during the ctest stage, several JSON test files are downloaded from an external repository. If policies forbid downloading artifacts during testing, you can download the files yourself and pass the directory with the test files via -DJSON_TestDataDirectory=path to CMake. Then, no Internet connectivity is required. See issue #2189 for more information.

Would using JSON_TestDataDirectory be an option?