unittests/tests-spiffs: fix dummy memory layout

Use 8 pages per sector to comply with spiffs requirements:
 - page_size must be block_size / (2^y) where y > 2

tests/unittests/tests-spiffs/tests-spiffs.c

@@ -29,10 +29,10 @@
 #else
 /* Test mock object implementing a simple RAM-based mtd */
 #ifndef SECTOR_COUNT
-#define SECTOR_COUNT 8
+#define SECTOR_COUNT 4
 #endif
 #ifndef PAGE_PER_SECTOR
-#define PAGE_PER_SECTOR 4
+#define PAGE_PER_SECTOR 8
 #endif
 #ifndef PAGE_SIZE
 #define PAGE_SIZE 128