Improve dictionary implementation test framework

[benoit-pierre]

Summary of changes

Add plover_build_utils.testing.dictionary_test helper, usage:

@dictionary_test
class TestMyDictionaryClass:

    # Mandatory: `implementation` class.
    DICT_CLASS = MyDictionaryClass

    # Mandatory: extension.
    DICT_EXTENSION = 'json'

    # Mandatory: valid sample dictionary contents.
    DICT_SAMPLE = b'{}'

    # Optional: if `True`, will check the implementation class
    # is registered as a valid `plover.dictionary` entrypoint.
    DICT_REGISTERED = True

    # Optional: if `False`, `len`, `__iter__`, and `items`
    # are not supported, and must respectively return:
    # 0 and empty sequences.
    # Note: only supported for read-only implementations,
    # writable implementations must support all sequence
    # methods.
    DICT_SUPPORT_SEQUENCE_METHODS = False

    # Optional: if `False`, then `reverse_lookup` is not
    # supported, and must return an empty set.
    DICT_SUPPORT_REVERSE_LOOKUP = False

    # Optional: if `False`, then `casereverse_lookup` is not
    # supported, and must return an empty set.
    # Note: if supported, then `DICT_SUPPORT_REVERSE_LOOKUP`
    # must be too.
    DICT_SUPPORT_CASEREVERSE_LOOKUP = False

    # Optional: load tests.
    DICT_LOAD_TESTS = (
        lambda: (
            # Dictionary file contents.
            b'''
            ''',
            # Expected entries, in Python-dictionary like format.
            '''
            "TEFT": 'test',
            'TEFT/-G': "testing",
            '''
        ),
    )

    # Optional: save tests.
    # Note: only for writable implementations.
    DICT_SAVE_TESTS = (
        lambda: (
            # Initial entries, in Python-dictionary like format.
            '''
            "TEFT": 'test',
            'TEFT/-G': "testing",
            '''
            # Expected saved dictionary contents, or `None`
            # to skip the byte-for-byte test of the resulting
            # file contents.
            b'''
            ''',
        ),
    )

    # Optional: if implemented, will be called to format
    # the contents before saving to a dictionary file,
    # including for load and save tests.
    @staticmethod
    def make_dict(self, contents):
        return contents

Fix 2 issues with StenoDictionary implementation:

• If the class implementation is marked as read-only, then loading from a writable file should still result in a read-only dictionary.
• Don't allow clear on a read-only dictionary.

Pull Request Checklist

• Changes have tests
• News fragment added in news.d. See documentation for details