Simplify tests.toml files

[ErikSchierboom]

The spec for tests.toml files has been updated: exercism/configlet#198 In short, we will no longer require include = true to be set, as this will be true for the vast majority of the test cases. We will allow test case entries to omit the include property, which we will then interpret as if being set to true. The main benefit of this is that the tests.toml files become more readable, which will make the exceptional cases (include = false) far easier to identify. See exercism/configlet#186 for the corresponding discussion.