Fix some sign-comparison warnings in tests

The new Catch2 version is structured so as to surface these warnings
that previously did not occur.  So they need to be resolved.

tests/list_test.cpp

@@ -301,7 +301,7 @@ TEST_CASE( "list insert and erase", "[list]" )
     }
 
     SECTION( "erase randomly till half empty" ) {
-        int count = 0;
+        size_t count = 0;
         do {
             for( cata::list<int>::iterator it = test_list.begin(); it != test_list.end(); ) {
                 if( ( xor_rand() & 7 ) == 0 ) {
@@ -316,7 +316,7 @@ TEST_CASE( "list insert and erase", "[list]" )
 
         CHECK( test_list.size() == 500000 - count );
 
-        for( int i = 0; i < count; i++ ) {
+        for( size_t i = 0; i < count; i++ ) {
             test_list.push_front( 1 );
         }
 
@@ -432,7 +432,7 @@ TEST_CASE( "list insert and erase", "[list]" )
         CHECK( prev_capacity != test_list.capacity() );
         CHECK( test_list.capacity() == 1000 );
 
-        int count = 0;
+        size_t count = 0;
         for( int loop1 = 0; loop1 < 50000; loop1++ ) {
             for( int loop = 0; loop < 10; loop++ ) {
                 if( ( xor_rand() & 7 ) == 0 ) {

tests/visitable_remove_test.cpp

@@ -392,7 +392,7 @@ TEST_CASE( "visitable_remove", "[visitable]" )
                 REQUIRE( count_items( sel, liquid_id ) == count - our );
             }
             THEN( "the correct number of items were removed" ) {
-                REQUIRE( del.size() == our );
+                REQUIRE( static_cast<int>( del.size() ) == our );
 
                 AND_THEN( "the removed items were all bottles" ) {
                     CHECK( std::all_of( del.begin(), del.end(), [&container_id]( const item & e ) {