patch 9.1.0351: No test that completing a partial mapping clears 'showcmd'

Problem:  No test that completing a partial mapping clears 'showcmd'.
Solution: Complete partial mappings in Test_showcmd_part_map() instead
          of using :echo.  Adjust some comments (zeertzjq).

closes: #14580

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_utf8.vim b/src/testdir/test_utf8.vim
index 3fd83cf..0f46240 100644
--- a/src/testdir/test_utf8.vim
+++ b/src/testdir/test_utf8.vim
@@ -300,7 +300,7 @@
   call StopVimInTerminal(buf)
 endfunc
 
-" When `setcellwidth` is used on characters that are not targets of `ambiwidth`.
+" Test setcellwidths() on characters that are not targets of 'ambiwidth'.
 func Test_setcellwidths_with_non_ambiwidth_character_dump()
   CheckRunVimInTerminal
 
@@ -321,7 +321,6 @@
   call StopVimInTerminal(buf)
 endfunc
 
-
 " For some reason this test causes Test_customlist_completion() to fail on CI,
 " so run it as the last test.
 func Test_zz_ambiwidth_hl_dump()