Add support for testing specific mutants

[Aimless321]

Currently, it is possible to specify which blocks of code need to be tested using mutation ranges, but there's no way to specify which individual mutants within those blocks should be tested.

Use case

In the Stryker Cloud Orchestrator, a mutation test run is split into tasks, where each task contains a set of mutants to be tested. These tasks are optimized based on test coverage to allow multiple mutants to be tested in the same task as long as there is no overlap in their test coverage.

To support this use case, it would be helpful to have a mechanism to specify exactly which mutants should be tested, rather than testing all mutants within a given code block.

Would it be possible to do this based on the mutatorName in combination with the current ranges?

[rouke-broersma]

In our use case mutant IDs are stable - for this specific use case, so we could also simply optionally allow mutant ID in MSP.

We could optionally allow mutant ID, to be used if the specific MSP server supports guaranteed stable mutant IDs when there are not changed to source code.

[Aimless321]

To add another example where the current implementation could cause issues.

These mutants are returned by a discover:

{
  "id": "1",
  "location": {
    "start": {
      "line": 7,
      "column": 41
    },
    "end": {
      "line": 9,
      "column": 4
    }
  },
  "description": null,
  "mutatorName": "BlockStatement",
  "replacement": "{}"
},
{
  "id": "2",
  "location": {
    "start": {
      "line": 8,
      "column": 32
    },
    "end": {
      "line": 8,
      "column": 64
    }
  },
  "description": null,
  "mutatorName": "ObjectLiteral",
  "replacement": "{}"
}

Passing only the file and location of the BlockStatement mutant, would also cause the 2nd mutant to be tested.

Perhaps it would make more sense to treat the locations passed to the mutationTest method as exact locations, instead of an area to search for mutants. Combined with the mutatorName i think that would be a consistent way of specifying a single mutant?