vim-patch:8.2.4867: listing of mapping with K_SPECIAL is wrong (#18419)

Problem:    Listing of mapping with K_SPECIAL is wrong.
Solution:   Adjust escaping of special characters. (closes vim/vim#10351)
ac402f4d64

Avoid overshadowing.
Cherry-pick Test_list_mapping() from Vim patches 8.2.{0148,2994}.
Fix rhs_is_noop.

src/nvim/testdir/test_mapping.vim

@@ -464,6 +464,40 @@ func Test_list_mappings()
   call assert_equal(['n  ,n            <Nop>'],
         \ execute('nmap ,n')->trim()->split("\n"))
 
+  " verbose map
+  call assert_match("\tLast set from .*/test_mapping.vim line \\d\\+$",
+        \ execute('verbose map ,n')->trim()->split("\n")[1])
+
+  " character with K_SPECIAL byte in rhs
+  nmap foo …
+  call assert_equal(['n  foo           …'],
+        \ execute('nmap foo')->trim()->split("\n"))
+
+  " modified character with K_SPECIAL byte in rhs
+  nmap foo <M-…>
+  call assert_equal(['n  foo           <M-…>'],
+        \ execute('nmap foo')->trim()->split("\n"))
+
+  " character with K_SPECIAL byte in lhs
+  nmap … foo
+  call assert_equal(['n  …             foo'],
+        \ execute('nmap …')->trim()->split("\n"))
+
+  " modified character with K_SPECIAL byte in lhs
+  nmap <M-…> foo
+  call assert_equal(['n  <M-…>         foo'],
+        \ execute('nmap <M-…>')->trim()->split("\n"))
+
+  " map to CTRL-V
+  exe "nmap ,k \<C-V>"
+  call assert_equal(['n  ,k            <Nop>'],
+        \ execute('nmap ,k')->trim()->split("\n"))
+
+  " map with space at the beginning
+  exe "nmap \<C-V> w <Nop>"
+  call assert_equal(['n  <Space>w      <Nop>'],
+        \ execute("nmap \<C-V> w")->trim()->split("\n"))
+
   nmapclear
 endfunc