check: disable HAVE_PRIVATENS by default

check: disable HAVE_PRIVATENS by default

Currently we have 3 ways to run a test case in _run_seq():

  if [ -n "${HAVE_PRIVATENS}" ]; then
      ./tools/run_privatens "./$seq"
      ...
  elif [ -n "${HAVE_SYSTEMD_SCOPES}" ]; then
      systemd-run --quiet --unit "${unit}" --scope \
             ./tools/run_setsid "./$seq" &
      ...
  else
      ./tools/run_setsid "./$seq" &
      ...
  fi

The "privatens" way brings in some regressions. We need more time
to develop and test this way, it's not time let it to be the
first default choice, so isolate the HAVE_PRIVATENS initialization
by a TRY_PRIVATENS parameter, and disable it by default.

Set TRY_PRIVATENS=yes to give "privatens" a try, otherwise run in
old ways. This patch can be removed after "privatens" way is stable.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>