add testcase for exit code handling

tests/osproc/texitcode.nim

@@ -0,0 +1,18 @@
+discard """
+  file: "texitcode.nim"
+  output: ""
+"""
+import osproc, os
+
+const filename = when defined(Windows): "tfalse.exe" else: "tfalse"
+
+doAssert fileExists(getCurrentDir() / "tests" / "osproc" / filename)
+
+var p = startProcess(filename, getCurrentDir() / "tests" / "osproc")
+doAssert(waitForExit(p) == QuitFailure)
+
+p = startProcess(filename, getCurrentDir() / "tests" / "osproc")
+var running = true
+while running:
+  running = running(p)
+doAssert(waitForExit(p) == QuitFailure)

tests/osproc/tfalse.nim

@@ -0,0 +1,2 @@
+import system
+quit(QuitFailure)