libgpiod: Show the output of failing tests in ptest log

run-ptest wrote each test's output into a file in the image, so an
intermittent gpiod-glib-test failure in testimage left nothing to debug.
Print the output when a test fails.

Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>

meta-oe/recipes-support/libgpiod/libgpiod-2.x/run-ptest

@@ -20,6 +20,8 @@ run_one_test() {
 
 	./$testbin > ./$testbin.out 2>&1
 	if [ $? -ne 0 ]; then
+		# keep the details of intermittent failures in the ptest log
+		cat ./$testbin.out
 		echo "FAIL: $testbin"
 	else
 		echo "PASS: $testbin"