tests: add helper script for processing doctests read from stdin

Writes stdin to a temp file and doctests it.

In the future we might want to spare the temp file and directly call into
doctest.

Also, with some tweaking it seems possible to adjust the line numbers reported
in an error report so they match the ones in the original file.

tests/heredoctest.py

@@ -0,0 +1,9 b''
+import doctest, tempfile, os, sys
+
+if __name__ == "__main__":
+    fd, name = tempfile.mkstemp(suffix='hg-tst')
+    os.write(fd, sys.stdin.read())
+    os.close(fd)
+    failures, _ = doctest.testfile(name, module_relative=False)
+    if failures:
+        sys.exit(1)