ExternalData: Add tests covering interfaces and errors

Add a Module.ExternalData test to verify data retrieval and test
argument DATA{} references.

Add a RunCMake.ExternalData test to verify error handling and automatic
transformation of a raw data to a content link and staged object.

Tests/Module/ExternalData/Data2/Data2Check.cmake

@@ -0,0 +1,9 @@
+if(NOT EXISTS "${Data2}")
+  message(SEND_ERROR "Input file:\n  ${Data2}\ndoes not exist!")
+endif()
+foreach(n 1 2 3)
+  string(REGEX REPLACE "_1_\\.my\\.dat$" "_${n}_.my.dat" SeriesCFile "${SeriesC}")
+  if(NOT EXISTS "${SeriesCFile}")
+    message(SEND_ERROR "Input file:\n  ${SeriesCFile}\ndoes not exist!")
+  endif()
+endforeach()