[git] Do a shallow clone of Tests/Data.

This reduces download times significantly as downloaded decreases
from over 1 GB to around 60 MB.

.gitmodules

@@ -16,6 +16,7 @@
 [submodule "Tests/Data"]
 	path = Tests/Data
 	url = https://github.com/ufz/ogs-data
+	shallow = true
 [submodule "ThirdParty/vtkGUISupportQt"]
 	path = ThirdParty/vtkGUISupportQt
 	url = https://github.com/ufz/vtkGUISupportQt.git