MDL-50611 test: use cURL extension to download Composer

The curl binary is nowhere near as common in Windows environments as it is
in Linux and Mac ones. In order to encourage more users to adopt Behat and
PHPUnit for their testing, we should avoid introducing unnecessary
hurdles.

lib/behat/classes/behat_command.php

@@ -157,7 +157,7 @@ class behat_command {
 
             // Returning composer error code to avoid conflicts with behat and moodle error codes.
             self::output_msg(get_string('errorcomposer', 'tool_behat'));
-            return BEHAT_EXITCODE_COMPOSER;
+            return TESTING_EXITCODE_COMPOSER;
         }
 
         // Behat test command.
@@ -167,7 +167,7 @@ class behat_command {
 
             // Returning composer error code to avoid conflicts with behat and moodle error codes.
             self::output_msg(get_string('errorbehatcommand', 'tool_behat', self::get_behat_command()));
-            return BEHAT_EXITCODE_COMPOSER;
+            return TESTING_EXITCODE_COMPOSER;
         }
 
         // No empty values.