trunner was not actually being tested in non-CTFFI mode; minor testament cleanups (#14377)

* use check
* trunner now works with cpp
* cleanup: move compiler/unittest_light => stdtest/unittest_light
* fix tests/readme.md
* remove deadcode references to rodfiles
* fix for windows

tests/stdlib/tosproc.nim

@@ -1,7 +1,6 @@
 # test the osproc module
 
-import stdtest/specialpaths
-import "$nim" / compiler/unittest_light
+import stdtest/[specialpaths, unittest_light]
 
 when defined(case_testfile): # compiled test file for child process
   from posix import exitnow

tests/stdlib/tstackframes.nim

@@ -1,5 +1,5 @@
 import std/[strformat,os,osproc]
-import "$nim/compiler/unittest_light" # works even if moved by megatest
+import stdtest/unittest_light
 
 proc main(opt: string, expected: string) =
   const nim = getCurrentCompilerExe()