PR46997: don't run clang-format on clang's testcases.

The formatting of the testcases matters and shouldn't be overwritten by
a tool.

Author: zygoloid

.arclint

@@ -6,6 +6,9 @@
       "script-and-regex.regex": "/^(?P<severity>[[:alpha:]]+)\n(?P<message>[^\n]+)\n(====|(?P<line>\\d),(?P<char>\\d)\n(?P<original>.*)>>>>\n(?P<replacement>.*)<<<<\n)$/s",
       "include": [
         "(\\.(cc|cpp|h)$)"
+      ],
+      "exclude": [
+        "(^clang/test/)"
       ]
     }
   }