Fix flaky primary key test

I noticed in a build that this test was flaky and causing subsequent
tests to fail with a no connection pool error. I don't entirely get why
this test was failing, it's not the only one where we remove a
connection. When this test fails, `@automatic_reconnect` is `false`.
After spending far too much time trying to figure out the combination of
tests that break this I decided to fix it by making a brand new
connection that we can remove. It's also a more accurate test because
we're asserting the connection is actually not `connected?`. Note that I
had to use the connection handler to remove the connection because there
is an inconsitency in remove_connection on the model. Unfortunately,
`NoConnection.remove_connection` will cause `connected?` to fall back to
`ActiveRecord::Base` which is contrary to the behavior of all other
methods on the active record models. I'm going to send a PR to fix that
but for now this should fix the (rare) flaky test for a primary key with
no connection.

To reproduce I used this seed: `SEED=29535 bundle exec rake test:mysql2`

activerecord/test/cases/primary_keys_test.rb

@@ -271,20 +271,26 @@ def test_serial_with_unquoted_sequence_name
 end
 
 class PrimaryKeyWithNoConnectionTest < ActiveRecord::TestCase
+  class NoConnection < ActiveRecord::Base
+    self.abstract_class = true
+    self.primary_key = "foo"
+  end
+
   self.use_transactional_tests = false
 
   unless in_memory_db?
     def test_set_primary_key_with_no_connection
-      connection = ActiveRecord::Base.remove_connection
-
-      model = Class.new(ActiveRecord::Base)
-      model.primary_key = "foo"
+      NoConnection.establish_connection :arunit
+      # execute to fully establish a connection
+      NoConnection.connection.execute("SELECT 1")
 
-      assert_equal "foo", model.primary_key
+      assert NoConnection.connected?
+      assert_equal "foo", NoConnection.primary_key
 
-      ActiveRecord::Base.establish_connection(connection)
+      ActiveRecord::Base.connection_handler.remove_connection_pool("PrimaryKeyWithNoConnectionTest::NoConnection")
+      assert_nil NoConnection.connected?
 
-      assert_equal "foo", model.primary_key
+      assert_equal "foo", NoConnection.primary_key
     end
   end
 end