Audit and fix up format strings

src/tests/test_utils.cc

@@ -38,7 +38,10 @@ void hexdump( const void *buf, size_t len, const char *name ) {
   const unsigned char *data = (const unsigned char *) buf;
   printf( DUMP_NAME_FMT, name );
   for ( size_t i = 0; i < len; i++ ) {
-    printf( "%02x", data[ i ] );
+    // Although data[i] is an unsigned char, it will be promoted to a signed int
+    // when passed as an argument. Explicitly cast it back to an unsigned type
+    // so it can be printed in hex.
+    printf( "%02x", static_cast<unsigned>( data[ i ] ) );
   }
   printf( "\n" );
 }