tests: subsys: fs: Add test for multiple file systems

Add a new test for testing multiple file systems mounted
simultaneously on Zephyr. The test enables FATFS and NFFS
togeather and uses RAM as backend storage device.

The intention of this test is to demonstrate multiple file systems
support and perform basic file and directory operations. This test
heavily reused the existing fat_fs_api and nffs_fs_api test sources.

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>