Replace Expectations with Assertions in spider tests

Prevent from throwing this warning in spider tests:

DEPRECATED: global use of must_be from
test/functional/webui/spider_test.rb:167. Use _(obj).must_be instead.
This will fail in Minitest 6.

src/api/test/functional/webui/spider_test.rb

@@ -164,7 +164,7 @@ def test_spider_anonymously
     crawl
     ActiveRecord::Base.clear_active_connections!
 
-    @pages_visited.keys.length.must_be :>, 800
+    assert_operator(@pages_visited.keys.length, :>, 800)
   end
 
   def test_spider_as_admin
@@ -175,6 +175,6 @@ def test_spider_as_admin
     crawl
     ActiveRecord::Base.clear_active_connections!
 
-    @pages_visited.keys.length.must_be :>, 1200
+    assert_operator(@pages_visited.keys.length, :>, 1200)
   end
 end