tests: (col) avoid hardcoding of errno string

tests: (col) avoid hardcoding of errno string

The col/multibyte test has a hardcoded error string as part of its
expected output that is returned by glibc's strerror(3P) function. Even
though many of these strings are the same across libc implementations,
they are not standardiced and some are certainly different. One example
is the string for EILSEQ on musl libc.

To fix this, we introduce a new test helper "test_strerror". The helper
can be invoked with an error code like "EILSEQ", which will cause it to
print out the the respective error message for that code. Note that
"test_strerror" cannot act on the error's value (e.g. 84 for EILSEQ), as
these aren't standardized either. Instead, we thus need to have an array
of the error's string representation ("EILSEQ") to its respective error
code (the define EILSEQ). The array can trivially be extended as
required, thus it is only sparsely populated with EILSEQ right now.

To fix the col/multibyte test, we introduce a call to sed(1) to replace
the strerror(3P) message from EILSEQ with "EILSEQ". Furthermore, as
we're running tests with the POSIX locale by default which treats all
bytes as valid multibyte sequences, we have to change to the C.UTF-8
locale instead to actually get an error.

Signed-off-by: Patrick Steinhardt <ps@pks.im>