teach clang that _fail() does not return.

cmocka's fail_msg() (_fail() in macro) cannot return. Clang's static analysis has a false null pointer dereference error because it does not know this.

	if (result != test->result) {
7
←
Assuming the condition is true	
→
8
←
Taking true branch	
→
102			fail_msg("# test \"%s\" on line %d: "
103				 "expected result %d (%s), got %d (%s)",
104				 test->id.description, test->id.lineno,
105				 test->result, isc_result_totext(test->result),
106				 result, isc_result_totext(result));
107		}
108	 
109		/*
110		 * Check output string if return value indicates success.
111		 */
112		if (result == ISC_R_SUCCESS && strcmp(*target, test->output) != 0) {
9
←
Assuming 'result' is equal to 0	
→
10
←
Null pointer argument in call to string comparison function
113			fail_msg("# test \"%s\" on line %d: "
114				 "expected output \"%s\", got \"%s\"",
115				 test->id.description, test->id.lineno,
116				 test->output, *target);
117		}