test: add "default" semantic to testme.json files

[svalat]

What

Adds the default semantic in the testme.json.

It will be usefull latter to also re-generate the reference data via the new pytest runner by knowing with which sets of parameter to run it (require a small extra work).

In addition it makes the `testme.json more readable on what is fixed and what vary.

Example

The testme.json now looks like :

{
  "default": {
    "dumpname": "dump.0001.dmp",
    "reconstruction": 2,
    "tolerance": 1e-14
  },
  "variants": {
     "mpi": [false, true],
     "single": [false, true],
  }
}

Doc

The documentation has been updated.