patch 8.2.0299: Vim9: ISN_STORE with argument not tested

Problem:    Vim9: ISN_STORE with argument not tested.  Some cases in tv2bool()
            not tested.
Solution:   Add tests.  Add test_unknown() and test_void().
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 4349e76..9ad88ed 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2864,6 +2864,8 @@
 test_null_list()		List	null value for testing
 test_null_partial()		Funcref	null value for testing
 test_null_string()		String	null value for testing
+test_unknown()			any	unknown value for testing
+test_void()			any	void value for testing
 test_option_not_set({name})	none	reset flag indicating option was set
 test_override({expr}, {val})	none	test with Vim internal overrides
 test_refcount({expr})		Number	get the reference count of {expr}