Add osproc stdin test.

tests/osproc/ta.nim

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+import strutils
+let x = stdin.readLine()
+echo x.parseInt + 5

tests/osproc/tstdin.nim

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+discard """
+  file: "tstdin.nim"
+  output: "10"
+"""
+import osproc, os, streams
+
+doAssert fileExists(getCurrentDir() / "tests" / "osproc" / "ta.exe")
+
+var p = startProcess("ta.exe", getCurrentDir() / "tests" / "osproc")
+p.inputStream.write("5\n")
+while true:
+  let line = p.outputStream.readLine()
+  if line != "":
+    echo line
+  else:
+    break