some test cleanups & category reorganization (#22010)

* clean up some test categories

* mention exact slice issue

* magics into system

* move trangechecks into overflow

* move tmemory to system

* try fix CI

* try fix CI

* final CI fix

tests/sets/tnewsets.nim

@@ -0,0 +1,6 @@
+# new test for sets:
+
+const elem = ' '
+
+var s: set[char] = {elem}
+assert(elem in s and 'a' not_in s and 'c' not_in s )