Skip part of a spec on MariaDB 10.5

This resulted in my MariaDB 10.5 installation raising an out of
memory error.

spec/integration/plugin_test.rb

@@ -2129,9 +2129,11 @@ def set(k, v, ttl) self[k] = v end
         proc{@ds.insert(try.merge(:m1=>'', :m2=>nil, :m3=>''))}.must_raise(Sequel::DatabaseError)
       end
 
-      # Test for dropping of constraint
-      @db.alter_table(:cv_test){validate{drop :maxl2}}
-      @ds.insert(@valid_row.merge(:minlen=>'1234567'))
+      unless @db.database_type == :mysql && @db.mariadb? && @db.server_version >= 10500
+        # Test for dropping of constraint
+        @db.alter_table(:cv_test){validate{drop :maxl2}}
+        @ds.insert(@valid_row.merge(:minlen=>'1234567'))
+      end
     end
 
     it "should set up automatic validations inside the model" do