tests: Replace calls to dd with native writeZeroes

tests/common.inc

@@ -72,5 +72,26 @@ sub waitForFile
 	return 0;
 }
 
+# writeZeroes($filename, $size)
+# Write zeroes of size $size to file $filename
+sub writeZeroes
+{
+        my $filename = shift;
+        my $size = shift;
+        open(my $fh, ">", $filename);
+        my $bs = 4096; # 4 KiB
+        my $block = "\0" x $bs;
+        my $remain;
+        for($remain = $size; $remain >= $bs; $remain -= $bs)
+        {
+                print($fh $block) or BAIL_OUT("Could not write to $filename: $!");
+        }
+        if($remain > 0)
+        {
+                $block = "\0" x $remain;
+                print($fh $block) or BAIL_OUT("Could not write to $filename: $!");
+        }
+}
+
 # As this file will be require()'d, it needs to return true
 return 1;

tests/normal.pl

@@ -88,7 +88,7 @@ sub corruption
 sub internalModification
 {
     $ofile = "$workingDir/crypt-internal-$$";
-    qx(dd if=/dev/urandom of=$ofile bs=2k count=2 2> /dev/null);
+    writeZeroes($ofile, 2*1024);
     ok(copy($ofile, "$crypt/internal"), "copying crypt-internal file");
 
     open(my $out1, "+<", "$crypt/internal");

tests/reverse.pl

@@ -129,7 +129,7 @@ sub grow {
 }
 
 sub largeRead {
-    system("dd if=/dev/zero of=$plain/largeRead bs=1M count=1 2> /dev/null");
+    writeZeroes("$plain/largeRead", 1024*1024);
     # ciphertext file name
     my $cname = encName("largeRead");
     # cfh ... ciphertext file handle