patch 8.2.4464: Dtrace files are recognized as filetype D

Problem:    Dtrace files are recognized as filetype D.
Solution:   Add a pattern for Dtrace files. (Teubel György, closes #9841)
            Add some more testing.

runtime/autoload/dist/ft.vim

@@ -206,6 +206,10 @@ export def EuphoriaCheck()
 enddef
 
 export def DtraceCheck()
+  if did_filetype()
+    # Filetype was already detected
+    return
+  endif
   var lines = getline(1, min([line("$"), 100]))
   if match(lines, '^module\>\|^import\>') > -1
     # D files often start with a module and/or import statement.