patch 8.2.1601: Vim9: cannot use 'true" with garbagecollect()

Problem:    Vim9: cannot use 'true" with garbagecollect().
Solution:   Use tv_get_bool(). (closes #6871)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 8651ef8..7cc323c 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1285,6 +1285,10 @@
   assert_equal(6, res)
 enddef
 
+def Test_garbagecollect()
+  garbagecollect(true)
+enddef
+
 def Test_insert_return_type()
   let l = insert([2, 1], 3)
   let res = 0