test: make the output of TEST-69 less painful to read

The logs from TEST-69 still contain a lot of unnecessary shell
metacharacters, so to make the output more readable let's just set
TERM=dumb, instead of having to strip everything semi-manually. Also,
move the related --background= tweak to TEST-69, since it's relevant
only for that particular test.

Follow-up for 8d4bfd38ed941aa8003d7007145eccc01f52a5f6.

v255-only change: --background= is not supported in v255's sd-nspawn, so
                  that hunk is dropped

(cherry picked from commit 8d9cdb31f7cc41bb2252be70a2410030551aabde)
(cherry picked from commit 93c5ff05dcad782acdf87f4491b053c708a4eb45)
(cherry picked from commit 75d7263080c1c373666a334b873443118f047af2)
(cherry picked from commit 8db3e00695ea262cccf80c7d862e3060b230087d)

test/test-shutdown.py

@@ -11,7 +11,6 @@ import pexpect
 
 
 def run(args):
-
     ret = 1
     logger = logging.getLogger("test-shutdown")
     logfile = None
@@ -24,7 +23,7 @@ def run(args):
 
     logger.info("spawning test")
     console = pexpect.spawn(args.command, args.arg, logfile=logfile, env={
-            "TERM": "linux",
+            "TERM": "dumb",
         }, encoding='utf-8', timeout=60)
 
     logger.debug("child pid %d", console.pid)