put std/threads under the umbrella of nimPreviewSlimSystem (#20711)

* put `std/threads` under the umbrella of `nimPreviewSlimSystem`

* add changelog

* fixes tests

* fixes tests again

* fixes tests

tests/stdlib/tenvvars.nim

@@ -7,7 +7,10 @@ discard """
 import std/envvars
 from std/sequtils import toSeq
 import stdtest/testutils
-import std/assertions
+import std/[assertions]
+
+when not defined(js):
+  import std/threads
 
 # "LATIN CAPITAL LETTER AE" in UTF-8 (0xc386)
 const unicodeUtf8 = "\xc3\x86"

tests/stdlib/tnetdial.nim

@@ -5,7 +5,7 @@ discard """
 """
 
 import os, net, nativesockets, asyncdispatch
-import std/[assertions]
+import std/[assertions, threads]
 
 ## Test for net.dial
 

tests/stdlib/tosenv.nim

@@ -7,7 +7,9 @@ discard """
 import std/os
 from std/sequtils import toSeq
 import stdtest/testutils
-import std/assertions
+
+when defined(nimPreviewSlimSystem):
+  import std/[assertions]
 
 # "LATIN CAPITAL LETTER AE" in UTF-8 (0xc386)
 const unicodeUtf8 = "\xc3\x86"
@@ -55,6 +57,8 @@ when defined(windows):
 proc c_getenv(env: cstring): cstring {.importc: "getenv", header: "<stdlib.h>".}
 
 when not defined(js) and not defined(nimscript):
+  when defined(nimPreviewSlimSystem):
+    import std/threads
   block: # bug #18533
     var thr: Thread[void]
     proc threadFunc {.thread.} = putEnv("foo", "fooVal2")

tests/stdlib/tssl.nim

@@ -4,7 +4,7 @@ discard """
   disabled: "openbsd"
 """
 # disabled: pending bug #15713
-import std/[net, nativesockets, assertions]
+import std/[net, nativesockets, assertions, threads]
 
 when defined(posix): import os, posix
 else: