Merge pull request #4828 from jfhg/unify_waitpid_handling

Unify waitpid handling

tests/osproc/tafalse.nim

@@ -0,0 +1,3 @@
+# 'tafalse.nim' to ensure it is compiled before texitcode.nim
+import system
+quit(QuitFailure)

tests/osproc/texitcode.nim

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+discard """
+  file: "texitcode.nim"
+  output: ""
+"""
+import osproc, os
+
+const filename = when defined(Windows): "tafalse.exe" else: "tafalse"
+let dir = getCurrentDir() / "tests" / "osproc"
+doAssert fileExists(dir / filename)
+
+var p = startProcess(filename, dir)
+doAssert(waitForExit(p) == QuitFailure)
+
+p = startProcess(filename, dir)
+var running = true
+while running:
+  running = running(p)
+doAssert(waitForExit(p) == QuitFailure)