Investigate test failure for test_show_function_definition_command

macOS build failed on 11/13. Marked as a `test teardown failure`. Stack trace:

```
self =
query = 'DROP DATABASE mssql_cli_db_c6b0809eb4aa45c3b63c44bd22934132;'

def execute_query(self, query):
# Try to run first as special command
try:
> for rows, columns, status, statement, is_error in special.execute(self, query):

mssqlcli/mssqlcliclient.py:95:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

mssqlcliclient =
sql = 'DROP DATABASE mssql_cli_db_c6b0809eb4aa45c3b63c44bd22934132;'

@export
def execute(mssqlcliclient, sql):
"""Execute a special command and return the results. If the special command
is not supported a KeyError will be raised.
"""
command, verbose, pattern = parse_special_command(sql)

if (command not in COMMANDS) and (command.lower() not in COMMANDS):
> raise CommandNotFound('Command not found: %s' % command)
E mssqlcli.packages.special.main.CommandNotFound: Command not found: DROP

mssqlcli/packages/special/main.py:63: CommandNotFound

During handling of the above exception, another exception occurred:

cls =

@classmethod
def tearDownClass(cls):
try:
# delete the database objects created
client = create_mssql_cli_client()
> list(client.execute_query('DROP DATABASE {0};'.format(cls.database)))

tests/test_special.py:41:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
mssqlcli/mssqlcliclient.py:108: in execute_query
for rows, columns, status, statement, is_error in self._execute_query(single_query):
mssqlcli/mssqlcliclient.py:115: in _execute_query
query_response, query_messages, query_had_error = self._execute_query_execute_request_for(query)
mssqlcli/mssqlcliclient.py:198: in _execute_query_execute_request_for
exit(1)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = Use exit() or Ctrl-D (i.e. EOF) to exit, code = 1

def __call__(self, code=None):
# Shells like IDLE catch the SystemExit, but listen when their
# stdin wrapper is closed.
try:
sys.stdin.close()
except:
pass
> raise SystemExit(code)
E SystemExit: 1

.tox/py36/lib/python3.6/site.py:397: SystemExit
```

tests/test_special.py と test_show_function_definition_command の setup および teardown を読み、その後 mssqlcli/mssqlcliclient.py から mssqlcli/packages/special/main.py まで追ってください。macOS の失敗を再現し、報告されている CommandNotFound および SystemExit エラーなしで teardown が完了することを確認してください。

索引モデルが issue の本文から書いたものです。