Fix boost unit-test dependency detection

[ranisalt]

Pull Request Prelude

• [x] I have followed proper The Forgotten Server code styling.
• [x] I have read and understood the contribution guidelines before making this PR.
• [x] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Running find_package twice with the same package and different components doesn't really work as we expected, at least for Boost. The second time it is called, to search for unit-test-framework, it reuses the same return value (which didn't request that component) and fails to find.

Solved by calculating the needed components before running find_package