Fix: Add support for the grains exercise

[ee7]

Previously, the tool did not create a tests.toml file for grains.

This PR is ugly, but it works as a hotfix.

Note that users will see the large numbers in scientific notation:

The following test case is missing:"
{
  "uuid": "c73b470a-5efb-4d53-9ac6-c5f6487f227b",
  "description": "64",
  "property": "square",
  "input": {
    "square": 64
  },
  "expected": 9.223372036854776e+16
}:

Do you want to include the test case ([y]es/[n]o/[s]kip)?:

and

The following test case is missing:"
{
  "uuid": "6eb07385-3659-4b45-a6be-9dc474222750",
  "description": "returns the total number of grains on the board",
  "property": "total",
  "input": {},
  "expected": 1.844674407370955e+17
}:

Do you want to include the test case ([y]es/[n]o/[s]kip)?:

Fixes: #39