[PATCH] add more tests for t-printf and t-strerror

[NIIBE Yutaka]

Hello,

pengyi <pengyi37 at huawei.com> wrote:
> Hi. I found in libgpg-error, the tests/t-strerror.c didn't include
> some system error case; the tests/t-printf.c didn't test the
> functionality of gpgrt_fwrite() which is also a common API.

Thank you.  It makes sense.

I applied your change, modified slightly, and pushed it.

Modifications are:

    * The string "[==>Niente<==]" is not relevant here,
      because it's specifically used to represent nullptr
      in string_filter function, but not here.
      So, I change it to the famous quote of Zen.

    * Function calls are formatted in GNU style (space before
      paren).

    * Adding missing comma for the variable LIST, using more
      familiar values of errors.

https://dev.gnupg.org/rE72e0fb3fc858301de7b200e1c08da6904ab44bc0

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