Deep dict type-checking x 5.

This commit is the epochal last commit in a commit chain deeply
type-checking the first key-value pair of each **mapping type hint**
(e.g., of the form `dict[..., ...]`, `collections.defaultdict[...,
...]`, `collections.abc.Mapping[..., ...]`,
`collections.abc.MutableMapping[..., ...]`,
`collections.abc.OrderedDict[..., ...]`, `typing.Dict[..., ...]`,
`typing.Mapping[..., ...]`, `typing.MutableMapping[..., ...]`, or
`typing.OrderedDict[..., ...]`) in `O(1)` time, partially resolving
long-standing feature requests #167 kindly submitted sometime during my
most recent past life by ardent typing fiend @langfield *and*
#2021 kindly submitted sometime during the past life immediately
preceding my most recent past life by Equinor ASA bear bro @jondequinor
(Jonas Grønås). Specifically, this commit exhaustively exercises this
type-checking with PEP 585-compliant type hints and sample data.
**BOOM**! (*Basking basilisks!*)

beartype_test/a00_unit/data/data_type.py

@@ -19,6 +19,7 @@
     Iterator,
 )
 from beartype._util.func.utilfuncmake import make_func
+from collections import defaultdict
 from contextlib import contextmanager
 from enum import Enum
 from functools import (
@@ -517,6 +518,32 @@ def function_module_name_none() -> None:
 Absolute filename of the current submodule, declared purely for convenience.
 '''
 
+# ....................{ DICTS                              }....................
+def _default_dict_str_factory() -> str:
+    '''
+    Arbitrary factory function returning arbitrary strings.
+    '''
+
+    return 'His steps to the sea-shore. A swan was there,'
+
+
+default_dict_int_to_str = defaultdict(_default_dict_str_factory)
+'''
+Non-empty default dictionary mapping integers to strings, initialized with an
+arbitrary factory function returning arbitrary strings.
+'''
+default_dict_int_to_str[0] = 'Beside a sluggish stream among the reeds.'
+default_dict_int_to_str[1] # == 'His steps to the sea-shore. A swan was there,'
+
+
+default_dict_str_to_str = defaultdict(_default_dict_str_factory)
+'''
+Non-empty default dictionary mapping strings to strings, initialized with an
+arbitrary factory function returning arbitrary strings.
+'''
+default_dict_str_to_str['His eyes pursued its flight.'] = (
+    'Thou hast a home,')
+
 # ....................{ SETS ~ callable                    }....................
 CALLABLES_PYTHON = frozenset((
     function,