patch 9.1.0711: tests: test_xxd may file when using different xxd

patch 9.1.0711: tests: test_xxd may file when using different xxd

Problem:  tests: test_xxd may file when using different xxd
Solution: Make Test_xxd_color_0 agnostic to xxd's path, similar to
          Test_xxd_color_1 by filtering out shell command prompt in
          screen dump file (James McCoy)

If an alternative xxd path is provided, e.g., the system installed xxd,
then the screen dump fails due to the difference in path.

From test_xxd.vim:
Found errors in Test_xxd_color2():
Run 1, 15:17:03 - 15:17:04:
command line..script /tmp/autopkgtest-lxc.1auv5tlk/downtmp/autopkgtest_tmp/vim/src/testdir/runtest.vim[607]..function RunTheTest[57]..Test_xxd_color2[36]..VerifyScreenDump line 67: See dump file difference: call term_dumpdiff("testdir/failed/Test_xxd_color_0.dump", "testdir/dumps/Test_xxd_color_0.dump"); difference in line 1: "|$+0&#ffffff0| |/|u|s|r|/|b|i|n|/|x@1|d| |-|R| |n|e|v|e|r| @1|<| |X@1|D|f|i|l|e|_|c|o|l|o|r|s| @33"

related: #15612

Signed-off-by: James McCoy <jamessan@jamessan.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>