selftests: timers: rtcpie: skip test if default RTC device does not exist

This test will require /dev/rtc0, the default RTC device, or one
specified by user to run. Since this default RTC is not guaranteed to
exist on all of the devices, so check its existence first, otherwise
skip this test with the kselftest skip code 4.

Without this patch this test will fail like this on a s390x zVM:
$ selftests: timers: rtcpie
$ /dev/rtc0: No such file or directory
not ok 1 selftests: timers: rtcpie # exit=22

With this patch:
$ selftests: timers: rtcpie
$ Default RTC /dev/rtc0 does not exist. Test Skipped!
not ok 9 selftests: timers: rtcpie # SKIP

Fixed up change log so "With this patch" text doesn't get dropped.
Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/timers/rtcpie.c

@@ -18,6 +18,8 @@
 #include <stdlib.h>
 #include <errno.h>
 
+#include "../kselftest.h"
+
 /*
  * This expects the new RTC class driver framework, working with
  * clocks that will often not be clones of what the PC-AT had.
@@ -35,8 +37,14 @@ int main(int argc, char **argv)
 	switch (argc) {
 	case 2:
 		rtc = argv[1];
-		/* FALLTHROUGH */
+		break;
 	case 1:
+		fd = open(default_rtc, O_RDONLY);
+		if (fd == -1) {
+			printf("Default RTC %s does not exist. Test Skipped!\n", default_rtc);
+			exit(KSFT_SKIP);
+		}
+		close(fd);
 		break;
 	default:
 		fprintf(stderr, "usage:  rtctest [rtcdev] [d]\n");