correct Ridge lambda factor 2 and add tests

[codecov[bot]]

Codecov Report

Patch coverage: 57.14% and project coverage change: +0.06 :tada:

Comparison is base (7884c71) 83.38% compared to head (0f3c11e) 83.45%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #27 +/- ## ========================================== + Coverage 83.38% 83.45% +0.06% ========================================== Files 8 8 Lines 1493 1499 +6 ========================================== + Hits 1245 1251 +6 Misses 248 248 ``` | [Impacted Files](https://codecov.io/gh/getzze/RobustModels.jl/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/linpred.jl](https://codecov.io/gh/getzze/RobustModels.jl/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2xpbnByZWQuamw=) | `83.21% <53.84%> (-3.82%)` | :arrow_down: | | [src/linresp.jl](https://codecov.io/gh/getzze/RobustModels.jl/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2xpbnJlc3Auamw=) | `78.14% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://codecov.io/gh/getzze/RobustModels.jl/pull/27/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.