Fix connection pool leaking between test cases

Author: Strech

spec/datadog/tracing/contrib/rails/support/base.rb

@@ -14,6 +14,11 @@
   end
 
   after do
+    # NOTE: We forsibly close connection pool to avoid leaking connection between
+    #       test cases.
+    #       This call is safe to be used on already closed connection pool.
+    application_record&.connection&.disconnect!
+
     # Reset references stored in the Rails class
     Rails.application = nil
     Rails.logger = nil