UTF8 ansi test is broken

[Dennisbonke]

While attempting with @64 to run our testsuite on the host libc to verify behavior, we found out that the utf8 test in the ansi category is broken. On a glibc host, all inputs of the test make mbrtowc return -1 and set errno to EILSEQ, while mlibc decodes them correctly. That seems to be a bug. I think we need to improve error handling in the test, in our mbrtowc and fix the test itself to actually include valid mbrtowc conversions.

CC: @avdgrinten as you wrote the internal decoding functions, mbrtowc and the utf8 test.

[64]

Related: #13