patch 8.2.0188: Check commands don't work well with Vim9 script

Problem:    Check commands don't work well with Vim9 script.
Solution:   Improve constant expression handling.
diff --git a/src/testdir/check.vim b/src/testdir/check.vim
index 30c4158..6efa348 100644
--- a/src/testdir/check.vim
+++ b/src/testdir/check.vim
@@ -1,10 +1,12 @@
 source shared.vim
 
+command -nargs=1 MissingFeature throw 'Skipped: ' .. <args> .. ' feature missing'
+
 " Command to check for the presence of a feature.
 command -nargs=1 CheckFeature call CheckFeature(<f-args>)
 func CheckFeature(name)
   if !has(a:name)
-    throw 'Skipped: ' .. a:name .. ' feature missing'
+    MissingFeature a:name
   endif
 endfunc