Add a C++ test to parse all synthetic test files

[nyurik]

We now have many synthetic tests in the test/synthetic/0x01 directory, consisting of a tiny MLT file, and a corresponding GeoJSON file. A test should parse all .mlt files, generate corresponding GeoJSON (in memory), and compare the result with the GeoJSON stored in the dir. Lastly, add this test to the CI.

Notes

• we use integer tile coordinates in the range 0..4095. Coordinates are allowed to be somewhat outside of that range, e.g. -42 or 4112 - to represent the surrounding buffer, or the value could be even higher e.g. for the tiles with larger extent
• geojson includes CRS section which can be safely ignored
• Layer name is added as a property to each feature

Similar JavaScript ticket: #847