MDL-55986 behat: Added option to optimize parallel runs by tags

run.php should only be used for running behat
before this patch if tags were passed, then behat.yml
was being re-written with features havings tags.
As this is not ideal, this option is now removed and
features with tags will be optimally split
by init.php when --optimize-run option is passed

admin/tool/behat/cli/util_single_run.php

@@ -48,6 +48,7 @@ list($options, $unrecognized) = cli_get_params(
         'tags'        => '',
         'updatesteps' => false,
         'run-with-theme' => false,
+        'optimize-runs' => '',
     ),
     array(
         'h' => 'help',
@@ -72,6 +73,7 @@ Options:
 --disable        Disables test environment
 --diag           Get behat test environment status code
 --updatesteps    Update feature step file.
+--optimize-runs  Split features with specified tags in all parallel runs.
 --run-with-theme Run all core features with specified theme.
 
 -h, --help Print out this help
@@ -178,7 +180,7 @@ if ($options['install']) {
     }
 
     // Enable test mode.
-    behat_util::start_test_mode($options['run-with-theme'], $parallel, $run);
+    behat_util::start_test_mode($options['run-with-theme'], $options['optimize-runs'], $parallel, $run);
 
     // This is only displayed once for parallel install.
     if (empty($run)) {