patch 9.1.1284: not possible to configure pum truncation char

Problem:  not possible to configure the completion menu truncation
          character
Solution: add the "trunc" suboption to the 'fillchars' setting to
          configure the truncation indicator (glepnir).

closes: #17006

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 4721477..a087629 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -203,8 +203,9 @@
       \ 'fileformats': [['', 'dos', 'dos,unix'], ['xxx']],
       \ 'fillchars': [['', 'stl:x', 'stlnc:x', 'vert:x', 'fold:x', 'foldopen:x',
       \		'foldclose:x', 'foldsep:x', 'diff:x', 'eob:x', 'lastline:x',
-      \		'stl:\ ,vert:\|,fold:\\,diff:x'],
-      \		['xxx', 'vert:']],
+      \		'trunc:_', 'trunc:_,eob:x,trunc:_',
+      \		'stl:\ ,vert:\|,fold:\\,trunc:…,diff:x'],
+      \		['xxx', 'vert:', 'trunc:', "trunc:\b"]],
       \ 'foldclose': [['', 'all'], ['xxx']],
       \ 'foldmethod': [['manual', 'indent', 'expr', 'marker', 'syntax', 'diff'],
       \		['', 'xxx', 'expr,diff']],