add testcase for #12005 (#17278)

* remove unnecessary when statement

* remove outdated codes

* reuse jsffi

* move js json coverage

* close #12005 add testcase

* Update tests/stdlib/tthreadpool.nim

* Update tests/stdlib/tthreadpool.nim

* Update tests/stdlib/tthreadpool.nim

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

tests/stdlib/tthreadpool.nim

@@ -0,0 +1,14 @@
+discard """
+  matrix: "--threads:on --gc:arc"
+  disabled: "freebsd"
+  output: "42"
+"""
+
+from std/threadpool import spawn, `^`, sync
+block: # bug #12005
+  proc doworkok(i: int) {.thread.} = echo i
+  spawn(doworkok(42))
+  sync() # this works when returning void!
+
+  proc doworkbad(i: int): int {.thread.} = i
+  doAssert ^spawn(doworkbad(42)) == 42 # bug was here