[tests] avoid -q option in dos2unix (for busybox/alpine compatibility)

tests/spec/preprocess_dos.sh

 #!/bin/sh
 gcc -C -E -I. -o $3 $2
-$1 -q $3
+# Note: some versions of dos2unix (e.g. Busybox) do not have a '-q' flag,
+# so we avoid using them
+$1 $3 >/dev/null 2>/dev/null