Feeds Tamper doesn't call submitConfigurationForm() on Tamper plugins

Problem/Motivation

When saving configuration for a Tamper plugin, Feeds Tamper doesn't call submitConfiguration() on it. Instead the configuration values are fetched directly from the form state, see this code from TamperFormBase:

$plugin_config = $form_state->getValue(self::VAR_PLUGIN_CONFIGURATION);
if ($plugin_config) {
  $config += $plugin_config;
}

For some Tamper plugins this causes issues and the config doesn't get saved in the right way. For example, this is an issue with the ConvertBoolean plugin as reported in #3280233: Tamper: "Convert to Boolean" If No Match.

Steps to reproduce

Proposed resolution

Make sure that submitConfigurationForm() gets called and add a test that ensures that it happens.

Remaining tasks

User interface changes

API changes

Data model changes