tests: drop mktemp(1) flags for better compatibility

$ mktemp --tmpdir tmp.XXXXXXXXXX.yuv
mktemp: illegal option -- -
usage: mktemp [-d] [-q] [-t prefix] [-u] template ...
       mktemp [-d] [-q] [-u] -t prefix
$ gmktemp --tmpdir tmp.XXXXXXXXXX.yuv
/tmp/tmp.ddQTBn4XmE.yuv

$ mktemp -t tmp.XXXXXXXXXX.yuv
/tmp/tmp.XXXXXXXXXX.yuv.SYYNp8Ki
$ gmktemp -t tmp.XXXXXXXXXX.yuv
/tmp/tmp.2ena2nKgv5.yuv

$ mktemp -t yuv
/tmp/yuv.rX2UAbax
$ gmktemp -t yuv
gmktemp: too few X's in template ‘yuv’

tests/util.sh

@@ -5,11 +5,11 @@
 set -eu${BASH+o pipefail}
 
 # Temporary files for encoder input and output.
-yuvfile="$(mktemp --tmpdir tmp.XXXXXXXXXX.yuv)"
+yuvfile="$(mktemp)"
-hevcfile="$(mktemp --tmpdir tmp.XXXXXXXXXX.hevc)"
+hevcfile="$(mktemp)"
 
 cleanup() {
-    rm -rf ${yuvfile} ${hevcfile}
+    rm -rf "${yuvfile}" "${hevcfile}"
 }
 trap cleanup EXIT
 
@@ -22,7 +22,7 @@ prepare() {
     cleanup
     print_and_run \
         ffmpeg -f lavfi -i "mandelbrot=size=${1}" \
-            -vframes "${2}" -pix_fmt yuv420p \
+            -vframes "${2}" -pix_fmt yuv420p -f yuv4mpegpipe \
             "${yuvfile}"
 }