make more standard libraries work with nimPreviewSlimSystem (#20343)

* make more standard libraries work with `nimPreviewSlimSystem`

* typo

* part two

* Delete specutils.nim

* fixes more tests

* more fixes

* fixes tests

* fixes three more tests

* add formatfloat import

* fix

* last

testament/lib/stdtest/specialpaths.nim

@@ -13,6 +13,8 @@ import compiler/nimpaths
 ]#
 
 import os
+when defined(nimPreviewSlimSystem):
+  import std/assertions
 
 # Note: all the const paths defined here are known at compile time and valid
 # so long Nim repo isn't relocated after compilation.

testament/lib/stdtest/unittest_light.nim

@@ -1,3 +1,6 @@
+import std/assertions
+
+
 proc mismatch*[T](lhs: T, rhs: T): string =
   ## Simplified version of `unittest.require` that satisfies a common use case,
   ## while avoiding pulling too many dependencies. On failure, diagnostic