Disable test parallelization on AppVeyor with opcache

This is too unstable due to shared opcache state. Disabling it until
this issue is resolved to reduce the number of spurious failures.

Author: nikic

.appveyor.yml

@@ -30,8 +30,10 @@ environment:
         matrix:
                 - THREAD_SAFE: 0
                   OPCACHE: 0
+                  PARALLEL: -j2
                 - THREAD_SAFE: 1
                   OPCACHE: 1
+                  PARALLEL:
                   INTRINSICS: AVX
 
 services:

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 -j2"
+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 %PARALLEL%"
 
 set EXIT_CODE=%errorlevel%