Fix PhutilUTF8TestCase::testUTF8Convert for PHP 8

Summary:
In PHP 8 passing an invalid encoding to mb_convert_encoding raises a
ValueError (which extends Error not Exception), so fix the test to also
catch Throwable (but leave the explicit Exception case for PHP 5, which
lacks Throwable).

Test Plan: Ran arc unit --everything

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21501

src/utils/__tests__/PhutilUTF8TestCase.php

@@ -498,6 +498,8 @@ final class PhutilUTF8TestCase extends PhutilTestCase {
       phutil_utf8_convert('xyz', 'moon language', 'UTF-8');
     } catch (Exception $ex) {
       $caught = $ex;
+    } catch (Throwable $ex) {
+      $caught = $ex;
     }
 
     $this->assertTrue((bool)$caught, pht('Conversion with bogus encoding.'));