Report the bignum word size in testbn.

I've found in the last day or two that the first thing I want to do
after any successful run of testbn is to check whether I was running
it with the right compile settings - so I should have made it easier
to find that out to begin with! Better late than never.

testbn.c

@@ -12,6 +12,7 @@
 #include <ctype.h>
 
 #include "ssh.h"
+#include "sshbn.h"
 
 void modalfatalbox(const char *p, ...)
 {
@@ -38,6 +39,8 @@ int main(int argc, char **argv)
     int line = 0;
     int passes = 0, fails = 0;
 
+    printf("BIGNUM_INT_BITS = %d\n", (int)BIGNUM_INT_BITS);
+
     while ((buf = fgetline(stdin)) != NULL) {
         int maxlen = strlen(buf);
         unsigned char *data = snewn(maxlen, unsigned char);