patch 9.0.1745: Missing test coverage for blockwise Visual highlight

Problem:  Missing test coverage for blockwise Visual highlight with
          virtual that starts with a double-width char.
Solution: Add a new virtual text to the test. Some other small fixes.

closes: #12835

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/src/testdir/test_highlight.vim b/src/testdir/test_highlight.vim
index 22d066f..6e39d4e 100644
--- a/src/testdir/test_highlight.vim
+++ b/src/testdir/test_highlight.vim
@@ -744,8 +744,8 @@
   let lines =<< trim END
 	call setline(1, 'The quick brown fox jumped over the lazy dogs')
   END
-  call writefile(lines, 'Xtest_colorcolumn_srb', 'D')
-  let buf = RunVimInTerminal('-S Xtest_colorcolumn_srb', {'rows': 10,'columns': 40})
+  call writefile(lines, 'Xtest_colorcolumn_sbr', 'D')
+  let buf = RunVimInTerminal('-S Xtest_colorcolumn_sbr', {'rows': 10,'columns': 40})
   call term_sendkeys(buf, ":set co=40 showbreak=+++>\\  cc=40,41,43\<CR>")
   call VerifyScreenDump(buf, 'Test_colorcolumn_3', {})