feat(fs): Add filesystem validation test suite and related configurations

Author: JakubAndrysek

tests/validation/fs/README.md

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+# Filesystem Validation Test Suite
+
+This test suite validates the functionality of different filesystem implementations available for ESP32: SPIFFS, FFat, and LittleFS.
+
+## Overview
+
+The test suite uses Unity testing framework to run a comprehensive set of filesystem operations across all three filesystem implementations. Each test is executed for each filesystem to ensure consistent behavior and identify filesystem-specific differences.
+
+## Tested Filesystems
+
+- **SPIFFS** (`/spiffs`) - SPI Flash File System
+- **FFat** (`/ffat`) - FAT filesystem implementation
+- **LittleFS** (`/littlefs`) - Little File System
+
+## Test Cases
+
+The suite includes the following test categories:
+
+- **Basic Operations**: File write, read, append
+- **Directory Operations**: Create directories, list files, nested directories
+- **File Management**: Rename, remove, exists checks
+- **Binary Operations**: Binary data write/read, seek operations
+- **Edge Cases**: Empty files, seek edge cases, file truncation
+- **Multiple Handles**: Concurrent file operations
+- **Space Tracking**: Free space monitoring
+- **Error Handling**: Non-existent file operations
+- **Large Files**: Operations with larger file sizes
+- **Write/Read Patterns**: Sequential and random access patterns
+- **Open Files Limit**: Maximum concurrent open files testing
+
+## Known Filesystem-Specific Behaviors
+
+### SPIFFS
+
+- **Directory Support**: SPIFFS does not have true directory support. Opening a directory always returns `true`, and closing it also always returns `true`, regardless of whether the directory actually exists or not. This is a limitation of the SPIFFS implementation.
+- **Error Handling**: Some error case tests are skipped for SPIFFS due to different error handling behavior compared to other filesystems.
+
+### LittleFS
+
+- **Open Files Limit**: LittleFS does not enforce a maximum open files limit at the same time. The `test_max_open_files_limit()` test is skipped for LittleFS as it doesn't have this constraint.
+
+### FFat
+
+- FFat follows standard FAT filesystem behavior and supports all tested operations including proper directory handling and open files limits.

tests/validation/fs/ci.yml

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+platforms:
+  qemu: false