Fix unitialized variable in ArcanistPhpunitTestResultParser

Summary:
This is to fix `arc unit` when running a test file with no test results (e.g. skipped)
```
EXCEPTION: (RuntimeException) Undefined variable: last_test_finished at [<phutil>/src/error/PhutilErrorHandler.php:210]
arcanist(head=master, ref.master=d54cb072facd), deviantart(), phutil(head=master, ref.master=75f675747648)
  #0 PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<arcanist>/src/unit/parser/ArcanistPhpunitTestResultParser.php:95]
  #1 ArcanistPhpunitTestResultParser::parseTestResults(string, string) called at [<deviantart>/unit/DaUnitEngine.php:150]
  #2 DaUnitEngine::parseTestResults(string, TempFile, string, string) called at [<deviantart>/unit/DaUnitEngine.php:82]
  #3 DaUnitEngine::run() called at [<arcanist>/src/workflow/ArcanistUnitWorkflow.php:186]
  #4 ArcanistUnitWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]
```

Test Plan: Create a test file with skipped tests. Run `arc unit`. Make sure the exception is not thrown.

Reviewers: joshuaspence, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin, aurelijus

Differential Revision: https://secure.phabricator.com/D13640

src/unit/parser/ArcanistPhpunitTestResultParser.php

@@ -33,6 +33,8 @@ final class ArcanistPhpunitTestResultParser extends ArcanistTestResultParser {
       $coverage = $this->readCoverage();
     }
 
+    $last_test_finished = true;
+
     $results = array();
     foreach ($report as $event) {
       switch (idx($event, 'event')) {