Make test generator check 'include' keys in tests.toml

[D3usXMachina]

As proposed in #204 (comment)

In accordance with https://github.com/exercism/docs/blob/main/building/configlet/sync.md, tests are not included if the include field in tests.toml is set to false.

In any other case, notably also if the there is no entry with the corresponding UUID in tests.toml, the test is included.

The include check can be disabled by supplying the -i or --ignore-toml flags to the generate.py script.

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[keiravillekode]

Thank you for improving the generator.