Optimize the multilanguage build process

Work In Progress!

This commit makes a rework of the build and rebuild process to better suit a multi-site setup.

This also includes a complete overhaul of the site tests. Previous these were a messy mix that
were testing just small parts of the build chain, some of it testing code-paths not even used in
"real life". Now all tests that depends on a built site follows the same and real production code path.

See #2309
Closes #2211
Closes #477
Closes #1744

source/filesystem.go

@@ -105,6 +105,9 @@ func (f *Filesystem) captureFiles() {
 
 	if err != nil {
 		jww.ERROR.Println(err)
+		if err == helpers.WalkRootTooShortError {
+			panic("The root path is too short. If this is a test, make sure to init the content paths.")
+		}
 	}
 
 }