Add test for deterministic signature generation

Deterministic signatures should be added in future versions of
Libtomcrypt, which is used by CryptX. Ticket link:
libtom/libtomcrypt#441

t/PrivateKey.t

@@ -1,7 +1,7 @@
 use strict;
 use warnings;
 
-use Test::More tests => 18;
+use Test::More tests => 19;
 use Try::Tiny;
 use Bitcoin::Crypto::Config;
 
@@ -31,6 +31,9 @@ my $pubkey = $privkey->getPublicKey();
 my $message = "Perl test script";
 my $signature = $privkey->signMessage($message);
 
+ok($privkey->signMessage($message) eq $signature, "Signatures generation should be deterministic")
+    or diag("Signatures generation seems to be nondeterministic, which is a possible private key security threat");
+
 ok($privkey->verifyMessage($message, $signature), "Valid signature");
 ok($pubkey->verifyMessage($message, $signature), "Pubkey recognizes signature");