Merge branch 'test/fatfs_test_app_readme' into 'master'

fatfs: add missing readme file for the test apps

See merge request espressif/esp-idf!20522

components/fatfs/test_apps/README.md

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+# fatfs component target tests
+
+This directory contains tests for `fatfs` component which are run on chip targets.
+
+See also [test_fatfs_host](../test_fatfs_host) directory for the tests which run on a Linux host.
+
+Fatfs tests can be executed with different `diskio` backends: `diskio_sdmmc` (SD cards over SD or SPI interface), `diskio_spiflash` (wear levelling in internal flash) and `diskio_rawflash` (read-only, no wear levelling, internal flash). There is one test app here for each of these backends:
+
+- [sdcard](sdcard/) — runs fatfs tests with an SD card over SDMMC or SDSPI interface
+- [flash_wl](flash_wl/) - runs fatfs test in a wear_levelling partition in SPI flash
+- [flash_ro](flash_ro/) - runs fatfs test in a read-only (no wear levelling) partition in SPI flash
+
+These test apps define:
+- test functions
+- setup/teardown routines
+- build/test configurations
+- pytest test runners
+
+The actual test cases (many of which are common between the test apps) are defined in the [test_fatfs_common component](test_fatfs_common/).