patch 9.0.1827: xxd: no color support

Problem:  xxd: no color support
Solution: Add color support using xxd -R

Add some basic color support for xxd

The hex-value and value are both colored with the same color depending
on the hex-value, e.g.:

    0x00 = white
    0xff = blue
    printable = green
    non-printable = red
    tabs and linebreaks = yellow

Each character needs 11 more bytes to contain color. (Same color in a
row could contain only one overhead but the logic how xxd creates colums
must be then changed.) Size of colored output is increased by factor of
~6. Also grepping the output will break when colors is used.

Flag for color is "-R", because less uses "-R".

Color uses parameters auto,always,never same as less and grep (among
others).

E.g.

xxd -R always $FILE | less -R

Add some screen-tests (that currently on work on linux) to verify the
feature works as expected.

closes: #12131

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Aapo Rantalainen <aapo.rantalainen@gmail.com>
diff --git a/src/testdir/term_util.vim b/src/testdir/term_util.vim
index 88e2b33..d376a95 100644
--- a/src/testdir/term_util.vim
+++ b/src/testdir/term_util.vim
@@ -56,6 +56,7 @@
 " "statusoff" - number of lines the status is offset from default
 " "wait_for_ruler" - if zero then don't wait for ruler to show
 " "no_clean" - if non-zero then remove "--clean" from the command
+" "cmd"  - run any other command, e.g. "xxd" (used in xxd test)
 func RunVimInTerminal(arguments, options)
   " If Vim doesn't exit a swap file remains, causing other tests to fail.
   " Remove it here.
@@ -90,7 +91,11 @@
     let reset_u7 = ' --cmd "set t_u7=" '
   endif
 
-  let cmd = GetVimCommandCleanTerm() .. reset_u7 .. a:arguments
+  if empty(get(a:options, 'cmd', ''))
+    let cmd = GetVimCommandCleanTerm() .. reset_u7 .. a:arguments
+  else
+    let cmd = get(a:options, 'cmd')
+  endif
 
   if get(a:options, 'no_clean', 0)
     let cmd = substitute(cmd, '--clean', '', '')
@@ -120,7 +125,7 @@
 
   call TermWait(buf)
 
-  if get(a:options, 'wait_for_ruler', 1)
+  if get(a:options, 'wait_for_ruler', 1) && empty(get(a:options, 'cmd', ''))
     " Wait for "All" or "Top" of the ruler to be shown in the last line or in
     " the status line of the last window. This can be quite slow (e.g. when
     " using valgrind).