tests: fix removed-directory test

tests: fix removed-directory test

The previous attempt to skip that test on NFS (commit 4181fc518362)
made the test fail; it introduced two problems:
a) In the good case, i.e., when the subshell returns with exit status 0,
the test ran into framework_failure_.
b) As the subshell also runs with 'set -x', the later comparison of
/dev/null with 'err' would fail.

* tests/ls/removed-directory.sh: Revert to the style without subshell,
and add 'test -d .' to verify that 'ls' can read the removed dir.