Testsuite, plugin: make testcase pattern more flexible

Testsuite, plugin: make testcase pattern more flexible

On Darwin, the message recorded in the sarif file contains:
  "message": {"text": "Segmentation fault: 11"}
which is different from, e.g., linux:
  "message": {"text": "Segmentation fault"}

Adjusting the testcase pattern to be a little more flexible.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/crash-test-write-though-null-sarif.c: Update
	expected pattern.