- fix null pointer dereference in VFS when VFS implementations are added
and removed in different order
- vfs_fat_sdmmc, vfs_fat_spiflash: pass correct drive to mkfs
(previously it would always do mkfs in the first drive)
- add test case
This change moves actual test code into test_fatfs_common.c and
refactors setup/teardown code to be contained within separate functions.
For each SDMMC FATFS test, identical test is added which can run with
WL partition in flash.
