Improve tests

(2nd attempt and 65be45e8 was reverted in 15ef0cbc)

Cleanup checkcode $C

Verify some error strings

Create test_dummy2.save in tests (and remove from repo)

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I found a nice writeup suggesting that printf has much better portability than echo

https://www.in-ulm.de/~mascheck/various/echo+printf/

https://unix.stackexchange.com/questions/65803/why-is-printf-better-than-echo

I don't have access to the gcc45 machine, so I'm not sure how to test (or verify if I break it after a merge) but printf has been part of coreutils since at least 2005 so I suspect it will work