[BUG] Response schema conformance failures are not properly deduplicated

[Stranger6667]

Describe the bug
When validating response conformance, the failures might not be deduplicated which creates a lot of noise.

To Reproduce
Steps to reproduce the behavior:

1. Build https://github.com/ajnisbet/opentopodata
2. Run Schemathesis against the following hand-written schema and supply base URL pointing to the running server:

schemathesis run schema.json --base-url=http://0.0.0.0:3000/v1 -c response_schema_conformance --hypothesis-phases=generate

Note the --hypothesis-phases=generate option, otherwise the test might take much longer

Schema:

{
  "openapi": "3.0.2",
  "info": {
    "title": "Test",
    "description": "Test",
    "version": "0.1.0"
  },
  "servers": [
    {
      "url": "http://0.0.0.0:5000/{basePath}",
      "variables": {
        "basePath": {
          "default": "v1"
        }
      }
    }
  ],
  "paths": {
    "/{dataset_name}": {
      "get": {
        "parameters": [
          {
            "name": "dataset_name",
            "in": "path",
            "required": true,
            "schema": {
              "type": "string",
              "enum": [
                "test-dataset"
              ]
            }
          },
          {
            "name": "locations",
            "in": "query",
            "required": true,
            "style": "pipeDelimited",
            "explode": false,
            "schema": {
              "type": "array",
              "minItems": 2,
              "maxItems": 2,
              "items": {
                "type": "number",
                "minimum": -180.0,
                "maximum": 180.0
              }
            }
          },
          {
            "name": "interpolation",
            "in": "query",
            "required": false,
            "schema": {
              "enum": [
                "nearest",
                "bilinear",
                "cubic"
              ]
            }
          },
          {
            "name": "nodata_value",
            "in": "query",
            "required": false,
            "schema": {
              "anyOf": [
                {
                  "type": "integer"
                },
                {
                  "enum": [
                    "null",
                    "nan"
                  ]
                }
              ]
            }
          }
        ],
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "application/json": {
                "schema": {
                  "type": "object",
                  "additionalProperties": false,
                  "properties": {
                    "status": {
                      "enum": ["OK"]
                    },
                    "results": {
                      "type": "array",
                      "items": {
                        "type": "object",
                        "additionalProperties": false,
                        "properties": {
                          "elevation": {"type": "integer", "nullable":  true}
                        }
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

3. Observe a lot of very similar failures in the output:

154. The received response does not conform to the defined schema!

Details: 

Additional properties are not allowed ('location' was unexpected)

Failed validating 'additionalProperties' in schema['properties']['results']['items']

Path parameters : {'dataset_name': 'test-dataset'}
Query           : {'locations': '99.92297954590147|-171.28327659808383', 'nodata_value': -1791173676829156192, 'interpolation': 'cubic'}

Run this Python code to reproduce this failure: 

    requests.get('http://0.0.0.0:3000/v1/test-dataset', headers={'User-Agent': 'schemathesis/2.8.4'}, params={'locations': '99.92297954590147|-171.28327659808383', 'nodata_value': -1791173676829156192, 'interpolation': 'cubic'})

Or add this option to your command line parameters: --hypothesis-seed=140907943817362640799165644218694011587


155. The received response does not conform to the defined schema!

Details: 

Additional properties are not allowed ('location' was unexpected)

Failed validating 'additionalProperties' in schema['properties']['results']['items']

Path parameters : {'dataset_name': 'test-dataset'}
Query           : {'locations': '98.18156716347|98.18156716347', 'interpolation': 'cubic', 'nodata_value': 17155926026267753522784395817154392287}

Expected behavior
Only one failure with the given jsonschema-level output is expected

Environment:

• OS: Linux
• Python version: 3.8.6
• Schemathesis version: 2.8.4
• Spec version: Open API 3.0.2

This issue is connected with #834, after implementing which this behavior should be fixed as well.

As an idea - difflib could help here