skip downloading VMTK test data during build

[JostMigenda]

Currently, while building VMTK it downloads the VMTK/tests/vmtk-test-data/ submodule, which contains about 500 MB of test images that we don’t use. This wastes bandwidth, disk space and slows the build process down; let’s change our build scripts (either CemrgApp/CMakeExternals/VMTK.cmake or within our VMTK fork) to skip this.

[OrodRazeghi]

makes very good sense, nice catch 👍