xorcism: remove rstest dependency (#1590)

rstest uses proc macros, which make the tests timeout due to long
compile times. Replace rstest with a custom declarative macro.

Brings test time from 7.5 seconds to 0.8 seconds on my machine.

Drawbacks:
* more indentation
* module structure of tests is flipped around

both of those seem minor to me. 

Although declarative macros can be hard to read for those unfamiliar, 
that was already somewhat the case with rstest's magic in my opinion. So
I personally don't think it's worse in terms of the students being able to
understand the tests.

The only other alternative I see is to disable the online tests 
altogether and leave a note about that in the exercise description. That
probably wouldn't be that bad, since people solving this hard exercise
most likely have a solid local setup. But it would still be cool to run
the tests online as well.

#1513

Author: senekor

exercises/practice/xorcism/Cargo.toml

@@ -11,5 +11,3 @@ io = []
 
 [dev-dependencies]
 hexlit = "0.5.0"
-rstest = "0.12.0"
-rstest_reuse = "0.1.0"