test: update conformance tests to fit the new format

[IlyaFaer]

Closes #92

Fields signingV4Tests and postPolicyV4Tests were added into url_signer_v4_test_data.json to separate signing test data from POST policies test data.

[IlyaFaer]

There are a lot of changes, but in fact I've only added signingV4Tests and postPolicyV4Tests into the json structure.

[IlyaFaer]

@frankyn, teardown failed in kokoro with 403 error:

==================================== ERRORS ====================================
_ ERROR at teardown of TestRetentionPolicy.test_bucket_w_default_event_based_hold _

self = <test_system.TestRetentionPolicy testMethod=test_bucket_w_default_event_based_hold>

    def tearDown(self):
        for bucket_name in self.case_buckets_to_delete:
            bucket = Config.CLIENT.bucket(bucket_name)
>           retry_429_harder(bucket.delete)()

tests/system/test_system.py:1709:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
test_utils/test_utils/retry.py:95: in wrapped_function
    return to_wrap(*args, **kwargs)
google/cloud/storage/bucket.py:1131: in delete
    client._connection.api_request(
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <google.cloud.storage._http.Connection object at 0x7f41c87a7970>
method = 'DELETE', path = '/b/w-def-ebh-1585825330597', query_params = {}
data = None, content_type = None, headers = None, api_base_url = None
api_version = None, expect_json = True, _target_object = None, timeout = 60

    def api_request(
        self,
        method,
        path,
        query_params=None,
        data=None,
        content_type=None,
        headers=None,
        api_base_url=None,
        api_version=None,
        expect_json=True,
        _target_object=None,
        timeout=_DEFAULT_TIMEOUT,
    ):
      ...
        if not 200 <= response.status_code < 300:
>           raise exceptions.from_http_response(response)
E           google.api_core.exceptions.Conflict: 409 DELETE https://storage.googleapis.com/storage/v1/b/w-def-ebh-1585825330597: The bucket you tried to delete was not empty.

.nox/system-3-8/lib/python3.8/site-packages/google/cloud/_http.py:423: Conflict
=================================== FAILURES ===================================
__________ TestRetentionPolicy.test_bucket_w_default_event_based_hold __________

self = <test_system.TestRetentionPolicy testMethod=test_bucket_w_default_event_based_hold>

    def test_bucket_w_default_event_based_hold(self):
        ...
        self.assertIsNone(other.retention_expiration_time)

>       blob.delete()

tests/system/test_system.py:1810:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
google/cloud/storage/blob.py:630: in delete
    self.bucket.delete_blob(
google/cloud/storage/bucket.py:1189: in delete_blob
    client._connection.api_request(
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <google.cloud.storage._http.Connection object at 0x7f41c87a7970>
method = 'DELETE', path = '/b/w-def-ebh-1585825330597/o/test-blob'
query_params = {'generation': 1585825333262407}, data = None
content_type = None, headers = None, api_base_url = None, api_version = None
expect_json = True, _target_object = None, timeout = 60

    def api_request(
        self,
        method,
        path,
        query_params=None,
        data=None,
        content_type=None,
        headers=None,
        api_base_url=None,
        api_version=None,
        expect_json=True,
        _target_object=None,
        timeout=_DEFAULT_TIMEOUT,
    ):
     ...

        if not 200 <= response.status_code < 300:
>           raise exceptions.from_http_response(response)
E           google.api_core.exceptions.Forbidden: 403 DELETE https://storage.googleapis.com/storage/v1/b/w-def-ebh-1585825330597/o/test-blob?generation=1585825333262407: Object 'w-def-ebh-1585825330597/test-blob' is under active Event-Based hold and cannot be deleted, overwritten or archived until hold is removed.

[frankyn]

Tear down should clear out all retention properties and then do bucket.delete(force=True) with retries.

Could you help improve the test code there?

[frankyn]

Separate PR for improving system tests would be much appreciated @IlyaFaer

[IlyaFaer]

Separate PR for improving system tests would be much appreciated @IlyaFaer

Sure, I'll see what I can do there
Opened an issue not to forget: #97