autotest.sh: Add PHPUNIT_EXE environment variable

Signed-off-by: Daniel Rudolf <github.com@daniel-rudolf.de>

autotest.sh

@@ -29,7 +29,10 @@ if [ -z "$PHP_EXE" ]; then
 	PHP_EXE=php
 fi
 PHP=$(which "$PHP_EXE")
-PHPUNIT=$(which phpunit)
+if [ -z "$PHPUNIT_EXE" ]; then
+    PHPUNIT_EXE=phpunit
+fi
+PHPUNIT=$(which "$PHPUNIT_EXE")
 
 set -e