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/stdlib/trat_init.nim

@@ -0,0 +1,14 @@
+discard """
+  output: '''true'''
+"""
+import rationals
+var
+  z = Rational[int](num: 0, den: 1)
+  o = initRational(num=1, den=1)
+  a = initRational(1, 2)
+
+try:
+  var
+    r = initRational(1, 0)  # this fails - no zero denominator
+except AssertionDefect:
+  echo "true"