Resolves mutant species keeping old organs they shouldn't have (#73728)

## About The Pull Request

regenerate organs never removed invalid organs when regenerating
## Why It's Good For The Game

Fixes https://github.com/tgstation/tgstation/issues/73640
adds a unit test to prevent regressions in the future

## Changelog
🆑
fix: Plasmamen don't have hearts, again
/🆑

code/modules/unit_tests/_unit_tests.dm

@@ -152,6 +152,7 @@
 #include "monkey_business.dm"
 #include "mouse_bite_cable.dm"
 #include "mutant_hands_consistency.dm"
+#include "mutant_organs.dm"
 #include "novaflower_burn.dm"
 #include "ntnetwork_tests.dm"
 #include "nuke_cinematic.dm"