patch 8.2.3856: Vim9: not enough tests

Problem:    Vim9: not enough tests.
Solution:   Run more expression tests also with Vim9. Fix an uncovered
            problem.
diff --git a/src/testdir/vim9.vim b/src/testdir/vim9.vim
index cb6e385..a6e1e67 100644
--- a/src/testdir/vim9.vim
+++ b/src/testdir/vim9.vim
@@ -185,32 +185,33 @@
 		           	 ->substitute('\<LSTART\>', '{', 'g')
 		           	 ->substitute('\<LMIDDLE\>', '->', 'g')
 				 ->substitute('\<LEND\>', '}', 'g')
+				 ->substitute('\<TRUE\>', '1', 'g')
+				 ->substitute('\<FALSE\>', '0', 'g')
 		           	 ->substitute('#"', ' "', 'g'))
   CheckLegacySuccess(legacylines)
 enddef
 
+def Vim9Trans(lines: list<string>): list<string>
+  return lines->mapnew((_, v) =>
+	    v->substitute('\<VAR\>', 'var', 'g')
+	    ->substitute('\<LET ', '', 'g')
+	    ->substitute('\<LSTART\>', '(', 'g')
+	    ->substitute('\<LMIDDLE\>', ') =>', 'g')
+	    ->substitute(' *\<LEND\> *', '', 'g')
+	    ->substitute('\<TRUE\>', 'true', 'g')
+	    ->substitute('\<FALSE\>', 'false', 'g'))
+enddef
+
 " Execute "lines" in a :def function, translated as in
 " CheckLegacyAndVim9Success()
 def CheckTransDefSuccess(lines: list<string>)
-  var vim9lines = lines->mapnew((_, v) =>
-  				v->substitute('\<VAR\>', 'var', 'g')
-		           	 ->substitute('\<LET ', '', 'g')
-		           	 ->substitute('\<LSTART\>', '(', 'g')
-		           	 ->substitute('\<LMIDDLE\>', ') =>', 'g')
-				 ->substitute(' *\<LEND\> *', '', 'g'))
-  CheckDefSuccess(vim9lines)
+  CheckDefSuccess(Vim9Trans(lines))
 enddef
 
 " Execute "lines" in a Vim9 script, translated as in
 " CheckLegacyAndVim9Success()
 def CheckTransVim9Success(lines: list<string>)
-  var vim9lines = lines->mapnew((_, v) =>
-  				v->substitute('\<VAR\>', 'var', 'g')
-		           	 ->substitute('\<LET ', '', 'g')
-		           	 ->substitute('\<LSTART\>', '(', 'g')
-		           	 ->substitute('\<LMIDDLE\>', ') =>', 'g')
-				 ->substitute(' *\<LEND\> *', '', 'g'))
-  CheckScriptSuccess(['vim9script'] + vim9lines)
+  CheckScriptSuccess(['vim9script'] + Vim9Trans(lines))
 enddef
 
 " Execute "lines" in a legacy function, :def function and Vim9 script.
@@ -218,6 +219,8 @@
 " Use 'LET' for an assignment
 " Use ' #"' for a comment
 " Use LSTART arg LMIDDLE expr LEND for lambda
+" Use 'TRUE' for 1 in legacy, true in Vim9
+" Use 'FALSE' for 0 in legacy, false in Vim9
 def CheckLegacyAndVim9Success(lines: list<string>)
   CheckTransLegacySuccess(lines)
   CheckTransDefSuccess(lines)