Properly push test artifacts

Cf. https://www.appveyor.com/docs/packaging-artifacts/#pushing-artifacts-from-scripts.

Closes GH-6990.

Author: cmb69

appveyor/test_task.bat

@@ -104,6 +104,6 @@ nmake test TESTS="%OPCACHE_OPTS% -q --offline --show-diff --show-slow 1000 --set
 
 set EXIT_CODE=%errorlevel%
 
-powershell -Command "$wc = New-Object 'System.Net.WebClient'; $wc.UploadFile('https://ci.appveyor.com/api/testresults/junit/%APPVEYOR_JOB_ID%', 'c:\junit.out.xml')"
+appveyor PushArtifact %TEST_PHP_JUNIT%
 
 exit /b %EXIT_CODE%