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Add documentation, comments. Move some 'Complex' sub-tests into 2 new 'Wrapping' and 'Testing' tests.

FIX: if the 'hostname' and 'nslookup' commands were not found from their HOSTNAME and NSLOOKUP cache definition, hard-coded values were used instead, thus causing pb if the corresponding progs were not in the PATH (RunCommand). Now use FindProgram() to be sure to find both, otherwise do nothing and set the site name to "unknown" (arbitrary. could be empty string ? or error ?).

Fix: "cd arg2 ; arg1" not working. OK with &&. Also should prevent from: "cd non_existing_dir_oops && rm -fr *"

Fix: create_file.cxx is not GENERATED, it exists (so that it can be removed, until SOURCE_FILES_REMOVE is synced with SOURCE_FILES)