Tests for `fileio` and `dirent`

[mlund]

Add tests for file I/O.

• fileio tested with cc65 and clang
• Fix dirent.s on clang and add test (i.e. attend the clang calling convention.
• Add Xemu unit test in tests/

[mlund]

@lydon42 would you be able to point me to an example where functions in dirent.h is used? I'd like to add a test to ensure that the llvm assembler part is in good order.

[lydon42]

MEGAFLASH does include it, but I think it only uses the struct. So mega65-core/src/utilities/qspicommon.c