Add test to ensure filename extensions are properly merged in the Update manager UI

This looks good to me. For a while, I was confused about 'update_test_archive' being the actual extension.

I interpret Dries's review as "needs work", in that we should make the bogus file extension sound more file extensiony. I agree that it wasn't at all obvious reading the test. How about just "foo", with a clarifying comment above that mentions this is a bogus extension for testing purposes?

On a more minor note, these:

+class UpdateInstallTestCase extends DrupalWebTestCase {
...
+  function testFileNameExtensionMerging() {

...need a line of PHPDoc.

Adds a test which passes. So why not :) ?

700558-6.update_manager_file_extensions_test.patch checks for 'zip', but 'zip' won't be found if zip_open() isn't available per system_archiver_info():

function system_archiver_info() {
  $archivers['tar'] = array(
    'class' => 'ArchiverTar',
    'extensions' => array('tar', 'tgz', 'tar.gz', 'tar.bz2'),
  );
  if (function_exists('zip_open')) {
    $archivers['zip'] = array(
      'class' => 'ArchiverZip',
      'extensions' => array('zip'),
    );
  }
  return $archivers;
}

Patch below changes the test to check for 'tar', which will always be displayed.

good catch. fixes fragile test - 'zip' is only displayed conditionally, but 'tar' is always an available option as per system_archiver_info().

Committed to HEAD. Thanks!