Enable parallel tests on AppVeyor

I'll be keeping an eye on spurious failures resulting from this...

Author: nikic

appveyor/test_task.bat

@@ -91,7 +91,7 @@ mkdir c:\tests_tmp
 set TEST_PHP_JUNIT=c:\junit.out.xml
 
 cd "%APPVEYOR_BUILD_FOLDER%"
-nmake test TESTS="%OPCACHE_OPTS% -q --offline --show-diff --show-slow 1000 --set-timeout 120 -g FAIL,XFAIL,BORK,WARN,LEAK,SKIP --temp-source c:\tests_tmp --temp-target c:\tests_tmp"
+nmake test TESTS="%OPCACHE_OPTS% -q --offline --show-diff --show-slow 1000 --set-timeout 120 -g FAIL,XFAIL,BORK,WARN,LEAK,SKIP --temp-source c:\tests_tmp --temp-target c:\tests_tmp -j2"
 
 set EXIT_CODE=%errorlevel%