Add a dedicated mechanism for disabling a test case

**Is your feature request related to a problem? Please describe.**
Let's say we wanna disable a testcase (because we are not interested in it anymore). The only way using the current API is to remove it and then add it as disabled. However, as it is indirect, it has potentially problematic consequences. For example, a new id will be given to the test case (which can cause the `parent_id` chain to break), or might delete some information (like in `OnDiskCorpus`)

**Describe the solution you'd like**
A dedicated API for disabling a test case.

**Describe alternatives you've considered**
Using `remove` and then `add_disabled`.
Also, `set_disabled` on the test case is not enough. It is not connected with the scheduler or the corpus, which may break the semantics (e.g., having a corpus with all disabled test cases is not considered empty).