patch 8.2.1798: Vim9: trinary operator condition is too permissive

Problem:    Vim9: trinary operator condition is too permissive.
Solution:   Use tv_get_bool_chk().
diff --git a/src/testdir/vim9.vim b/src/testdir/vim9.vim
index 2e4b03f..aeeb5a3 100644
--- a/src/testdir/vim9.vim
+++ b/src/testdir/vim9.vim
@@ -52,3 +52,10 @@
   CheckDefFailure(lines, error, lnum)
   CheckScriptFailure(['vim9script'] + lines, error, lnum + 1)
 enddef
+
+" Check that a command fails both when executed in a :def function and when
+" used in Vim9 script.
+def CheckDefExecAndScriptFailure(lines: list<string>, error: string, lnum = -3)
+  CheckDefExecFailure(lines, error, lnum)
+  CheckScriptFailure(['vim9script'] + lines, error, lnum + 1)
+enddef