test: Update tests to use the skip feature

Some tests currently return 0 when they want to be skipped. Update them to
return -EAGAIN instead, so they are counted as skipped.

A few tests are in two parts, with the latter part being skipped in
certain situations. Split these into two and use the correct condition for
the second part.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/of_platdata.c b/test/dm/of_platdata.c
index 7af798b..a241c42 100644
--- a/test/dm/of_platdata.c
+++ b/test/dm/of_platdata.c
@@ -150,7 +150,7 @@
 
 	/* Skip this test if there is no platform data */
 	if (!CONFIG_IS_ENABLED(OF_PLATDATA_DRIVER_RT))
-		return 0;
+		return -EAGAIN;
 
 	/* Record the indexes that are found */
 	memset(found, '\0', sizeof(found));