test(xinetd_services): mock a proper /etc/xinetd.d

To test `_comp_compgen_xinetd_services`, we have been using a
directory /test/fixtures/shared/bin (which contained two files `arp`
and `ifconfig`as a mock /etc/xinetd.d.  However, the directory
/test/fixtures/shared/bin is shared with other tests, and the contents
of the files therein are not proper xinetd configurations.  We want to
prepare a separate directory for a mock /etc/xinetd.d.  This patch
adds it under /test/fixtures/_comp_compgen_xinetd_services/xinetd.d.

Author: akinomyoga

test/fixtures/_comp_compgen_xinetd_services/xinetd.d/arp

@@ -17,7 +17,7 @@ def test_env_non_pollution(self, bash):
     def test_basic(self, bash):
         output = assert_bash_exec(
             bash,
-            "foo() { local _comp__test_xinetd_dir=$PWD/shared/bin; unset -v COMPREPLY; "
+            "foo() { local _comp__test_xinetd_dir=$PWD/_comp_compgen_xinetd_services/xinetd.d; unset -v COMPREPLY; "
             '_comp_compgen_xinetd_services; printf "%s\\n" "${COMPREPLY[@]}"; }; foo; unset -f foo',
             want_output=True,
         )