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

tests/stdlib/tfdleak.nim

@@ -7,6 +7,9 @@ discard """
 
 import os, osproc, strutils, nativesockets, net, selectors, memfiles,
        asyncdispatch, asyncnet
+
+import std/[assertions, syncio]
+
 when defined(windows):
   import winlean
 
@@ -56,7 +59,7 @@ proc isValidHandle(f: int): bool =
 proc main() =
   if paramCount() == 0:
     # Parent process
-    let f = system.open("__test_fdleak", fmReadWrite)
+    let f = syncio.open("__test_fdleak", fmReadWrite)
     defer: close f
 
     leakCheck(f.getOsFileHandle, "system.open()")