selftests/powerpc: Skip test instead of failing

Current core-pkey selftest fails if the test runs without privileges to
write into the core pattern file (/proc/sys/kernel/core_pattern). This
causes the test to fail and give the impression that the subsystem being
tested is broken, when, in fact, the test is being executed without the
proper privileges. This is the current error:

	test: core_pkey
	tags: git_version:v4.19-3-g9e3363be9bce-dirty
	Error writing to core_pattern file: Permission denied
	failure: core_pkey

This patch simply skips this test if it runs without the proper privileges,
avoiding this undesired failure.

CC: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
CC: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/ptrace/core-pkey.c

@@ -352,10 +352,7 @@ static int write_core_pattern(const char *core_pattern)
 	FILE *f;
 
 	f = fopen(core_pattern_file, "w");
-	if (!f) {
-		perror("Error writing to core_pattern file");
-		return TEST_FAIL;
-	}
+	SKIP_IF_MSG(!f, "Try with root privileges");
 
 	ret = fwrite(core_pattern, 1, len, f);
 	fclose(f);