nixos tests: retry: Count down to 0, and pass remaining attempts to the sub

Allows test functions to output diagnostic information on failure.

nixos/lib/test-driver/Machine.pm

···
 sub retry {
     my ($coderef) = @_;
     my $n;
-    for ($n = 0; $n < 900; $n++) {
-        return if &$coderef;
+    for ($n = 899; $n >=0; $n--) {
+        return if &$coderef($n);
         sleep 1;
     }
     die "action timed out after $n seconds";