Review Poetry detector tests

[JamieMagee]

In #184 and #230 we migrated from Nett to Tomlyn as our TOML parsing library as Nett was unsupported and deprecated. We used the IgnoreMissingProperties option to allow us to parse poetry.lock files which are missing properties defined in our PoetryLock model. Unfortunately, this property was not being respected for objects, like metadata.

This issue was fixed in https://github.com/xoofx/Tomlyn/pull/44, and upgrading to Tomlyn version 0.15.1 should resolve the issue. However, this should have been caught by our unit tests.

This issue is created to followup on why this was not caught by our tests cases.

See #301 for more context