Add tests for the mesh reader

qssgmesh.cpp parses a format with no tests at all, so this adds a suite
before touching it. MeshBuilder assembles containers byte by byte from
the format description rather than by mirroring the reader, which is
what lets it catch a wrong reading of the format.

Covered: a built mesh compared field by field for every version, the
subset record growing twice between versions 3 and 6, entry names of
every length modulo four, the index component types, multi mesh
containers, and save() and loadMesh() round trips.

Also adds a libFuzzer target and the data/fuzz directory the suite picks
findings up from.

Change-Id: I5ac616c22e1e3fdaa822c91a2e84c127d8a6a6da
Reviewed-by: Christian Strømme <christian.stromme@qt.io>