patch 8.2.0224: compiling :elseif not tested yet

Problem:    compiling :elseif not tested yet.
Solution:   Add test for :elseif.  Fix generating jumps.

src/testdir/test_vim9_script.vim

@@ -459,34 +459,22 @@ def do_something():
 EOF
 endfunc
 
-def HasEval()
-  if has('eval')
-    echo 'yes'
+def IfElse(what: number): string
+  let res = ''
+  if what == 1
+    res = "one"
+  elseif what == 2
+    res = "two"
   else
-    echo 'no'
+    res = "three"
   endif
+  return res
 enddef
 
-def HasNothing()
-  if has('nothing')
-    echo 'yes'
-  else
-    echo 'no'
-  endif
-enddef
-
-def Test_compile_const_expr()
-  assert_equal("\nyes", execute('call HasEval()'))
-  let instr = execute('disassemble HasEval')
-  assert_match('PUSHS "yes"', instr)
-  assert_notmatch('PUSHS "no"', instr)
-  assert_notmatch('JUMP', instr)
-
-  assert_equal("\nno", execute('call HasNothing()'))
-  instr = execute('disassemble HasNothing')
-  assert_notmatch('PUSHS "yes"', instr)
-  assert_match('PUSHS "no"', instr)
-  assert_notmatch('JUMP', instr)
+def Test_if_elseif_else()
+  assert_equal('one', IfElse(1))
+  assert_equal('two', IfElse(2))
+  assert_equal('three', IfElse(3))
 enddef