don't establish a new connection when testing with sqlite3_mem.

This fixes broken `rake test_sqlite3_mem` suite for Active Record.
The problem is that that the old database with the schema is lost
when establishing a new connection. Upon reconnting we are left
with a blank database and tests down the line start failing.

activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb

@@ -34,6 +34,8 @@ module ActiveRecord
       end
 
       def test_connect_with_url
+        skip "can't establish new connection when using memory db" if in_memory_db?
+        begin
           original_connection = ActiveRecord::Base.remove_connection
           tf = Tempfile.open 'whatever'
           url = "sqlite3://#{tf.path}"
@@ -44,8 +46,11 @@ module ActiveRecord
           tf.unlink
           ActiveRecord::Base.establish_connection(original_connection)
         end
+      end
 
       def test_connect_memory_with_url
+        skip "can't establish new connection when using memory db" if in_memory_db?
+        begin
           original_connection = ActiveRecord::Base.remove_connection
           url = "sqlite3:///:memory:"
           ActiveRecord::Base.establish_connection(url)
@@ -53,6 +58,7 @@ module ActiveRecord
         ensure
           ActiveRecord::Base.establish_connection(original_connection)
         end
+      end
 
       def test_valid_column
         column = @conn.columns('items').find { |col| col.name == 'id' }