[Meta] Improve unit test coverage

[ed255]

We can improve our unit test coverage in order to reduce the possibility of introducing bugs while refactoring and optimizing the code (as it happened in https://github.com/privacy-scaling-explorations/halo2/issues/321)

Here are some tasks that we can do

• https://github.com/privacy-scaling-explorations/halo2/pull/328 Add a tool to check the current unit test coverage
• https://github.com/privacy-scaling-explorations/halo2/pull/333. Coverage to 80%.
• https://github.com/privacy-scaling-explorations/halo2/pull/349. Deterministic tests to check if anything gone wrong.
• Based on the results of the tool, introduce tests that cover the different paths that the code can take
• Add more negative tests (circuits that have constraints / assignments that are supposed to not pass)
  • For example, a lookup to a table where the entry doesn't exists (see https://github.com/privacy-scaling-explorations/halo2/issues/325#issuecomment-2107517765)