tests: Use memstream for config-parser test

Using real files is unnecessarily heavy and error-prone. Fixes timeouts
seen on CI with ASan.

Signed-off-by: Daniel Stone <daniels@collabora.com>

include/libweston/config-parser.h

@@ -88,6 +88,9 @@ weston_config_section_get_bool(struct weston_config_section *section,
 const char *
 weston_config_get_name_from_env(void);
 
+struct weston_config *
+weston_config_parse_fp(FILE *file);
+
 struct weston_config *
 weston_config_parse(const char *name);