tests: fat_fs_api: fix main-stack overflow for the RAM-disk case

- Fixes the main stack overflow for the fat_fs_api
  RAM disk configuration on real platforms.
- Sets CONFIG_MAIN_STACK_SIZE=3072. By default it is 1KB,
  but the stack usage is about 2KB.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>