tests/extmod/vfs_fat_ilistdir_del.py: Use 512-byte erase block size.

Following other vfs_fat tests, so the test works on ports like stm32 that
only support 512-byte block size.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/vfs_fat_ilistdir_del.py

@@ -11,7 +11,7 @@ except (ImportError, AttributeError):
 
 
 class RAMBlockDevice:
-    ERASE_BLOCK_SIZE = 4096
+    ERASE_BLOCK_SIZE = 512
 
     def __init__(self, blocks):
         self.data = bytearray(blocks * self.ERASE_BLOCK_SIZE)
@@ -72,7 +72,7 @@ def test(bdev, vfs_class):
 
 
 try:
-    bdev = RAMBlockDevice(30)
+    bdev = RAMBlockDevice(50)
 except MemoryError:
     print("SKIP")
     raise SystemExit