[FIX] Prevent infinite input loop when testing with readline in non-interactive mode

[itislu]

It would look like this:

Also:

• Let readline print to stderr (same as bash).
  Bash's behavior can be tested with bash -i 2>/dev/null.
• Print welcome msg to stderr for consistency with readline.

[itislu]

The test failed because after the changes to the tester it needs the following change: 918116e

• So first merge [CI] Use a reusable workflow for regression test #368

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Oh I was wrong, the necessary change mentioned above was just so we can see the output of valgrind.
But for some reason, the valgrind trace-children-skip doesn't work for one test case anymore...

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• Fixed with 3331ad7 in [FIX] Fix compatibility with manual prompt printout 42_minishell_tester#25