Tests using XDG_DATA_HOME are not working on Darwin

Darwin dirs.m handling is redirecting VLC_USERDATA_DIR to hardcoded values:

char *config_GetUserDir (vlc_userdir_t type)
{
    const char *psz_path;
    switch (type) {
        case VLC_CONFIG_DIR:
        case VLC_TEMPLATES_DIR:
        case VLC_USERDATA_DIR:
        case VLC_CACHE_DIR:
            return getAppDependentDir(type);

static char *getAppDependentDir(vlc_userdir_t type)
{
    NSString *formatString;
    switch (type) {
        case VLC_CONFIG_DIR:
            formatString = @"%s/Library/Preferences/%@";
            break;
        case VLC_TEMPLATES_DIR:
        case VLC_USERDATA_DIR:
            formatString = @"%s/Library/Application Support/%@";
            break;

But the tests (test_modules_lua_extensions and test_modules_misc_medialibrary) want to override this value when running, to configure the runtime environment.

There are three solutions:

• handle the XDG vars like on POSIX.
• add a new interception mechanism for darwin.
• setup HOME as well.