libgpiod: redirect stdout from the ptest executable to a file

Most tests redirect their standard output to a file. Let's make libgpiod
consistent with this pattern.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-support/libgpiod/files/run-ptest

@@ -4,7 +4,7 @@ testbin="gpiod-test"
 ptestdir=$(dirname "$(readlink -f "$0")")
 cd $ptestdir/tests
 
-./$testbin
+./$testbin > ./$testbin.out
 if [ $? -ne 0 ]; then
 	echo "FAIL: $testbin"
 else