Revert "[travis][appveyor] don't cache .phpunit"

This reverts commit 5182253aa2.

.appveyor.yml

@@ -4,6 +4,7 @@ clone_folder: c:\projects\symfony
 
 cache:
     - composer.phar
+    - .phpunit -> phpunit
 
 init:
     - SET PATH=c:\php;%PATH%

.travis.yml

@@ -33,6 +33,7 @@ matrix:
 
 cache:
     directories:
+        - .phpunit
         - php-$MIN_PHP
         - ~/php-ext
 

phpunit

@@ -1,6 +1,8 @@
 #!/usr/bin/env php
 <?php
 
+// Cache-Id: 2020-04-10 20:30 UTC
+
 if (!file_exists(__DIR__.'/vendor/symfony/phpunit-bridge/bin/simple-phpunit')) {
     echo "Unable to find the `simple-phpunit` script in `vendor/symfony/phpunit-bridge/bin/`.\nPlease run `composer update` before running this command.\n";
     exit(1);