[NDB_BVL_Instrument_LINST] Show subtests (#6116)

Linst instrument subtests are blank and not able to render its fields. This is because of legacy request variable left over from before PR #4229 that introduced a new instruments loading module.

php/libraries/NDB_BVL_Instrument_LINST.class.inc

@@ -408,7 +408,7 @@ class NDB_BVL_Instrument_LINST extends \NDB_BVL_Instrument
             // parse the whole file for table{@} and other meta elements
             $addElements = true;
             if (!empty($this->page)) {
-                $currentPage = $_REQUEST['subtest'];
+                $currentPage = $this->page;
                 $addElements = false;
             } else {
                 $currentPage = 'top';