Redirect to the currencies page after submitting the import form

#PR: https://github.com/drupalcommerce/commerce/pull/569

Great! Made a comment on the PR. It'd be great to add a test assertion that the collection route is the current page after submitting the form.

  public function testCurrencyImport() {
    $this->drupalGet('admin/commerce/config/currency/import');
    $edit = [
      'currency_codes[]' => ['CHF'],
    ];
    $this->submitForm($edit, 'Import');
    // INSERT ROUTE CHECK HERE
    $currency = Currency::load('CHF');

CurrencyTest::testCurrencyImport test extended to check that he collection route is the current page after submitting the form. PR updated accordingly.

Committed in acae42b