tests/extmod/vfs_fat_fileio2.py: Close test file at end of test.

Otherwise it can lead to inconsistent results running subsequent tests.
Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/vfs_fat_fileio2.py

@@ -112,3 +112,4 @@ try:
     f.write(bytearray(bsize * free))
 except OSError as e:
     print("ENOSPC:", e.args[0] == 28)  # uerrno.ENOSPC
+f.close()