test-driver: Set the date to the current time on boot

Should allow merging #1816

nixos/lib/test-driver/test-driver.pl

···
     my ($args) = @_;
     my $vm = Machine->new({%{$args}, log => $log, redirectSerial => ($ENV{USE_SERIAL} // "0") ne "1"});
     $vms{$vm->name} = $vm;
+    $vm->execute("date -s @" . time);
     return $vm;
 }