fio-tests: add comprehensive filesystem testing documentation

The filesystem testing feature added in the previous commit lacked user
documentation. This adds comprehensive documentation covering all aspects
of the filesystem testing capabilities including support for declared
hosts.

Created a dedicated filesystem testing documentation file that covers the
supported filesystems including XFS with configurable block sizes from 4K
to 64K with features like reflink and rmapbt, ext4 with both standard and
bigalloc configurations, and btrfs with modern features including
compression options.

The documentation explains the architecture including the third drive
usage for filesystem testing separate from block device testing, the
automatic filesystem lifecycle management, and integration with the
existing test matrix capabilities.

Detailed quick start examples are provided for single filesystem testing,
multi-filesystem comparison scenarios, cross-filesystem comparisons, and
comprehensive XFS block size analysis. Each example includes the complete
workflow from configuration through result collection.

The CLI override support is thoroughly documented showing how to use
environment variables like FIO_TESTS_QUICK_TEST and FIO_TESTS_RUNTIME to
enable rapid iteration without reconfiguration. This is particularly
useful for development workflows and CI integration where quick validation
is needed.

Multi-filesystem testing architecture is explained including the
section-based approach, node generation patterns, Ansible group
organization, and the section configuration file format. This helps users
understand how to extend the system with custom filesystem configurations.

Results and analysis documentation covers the result collection process,
multi-filesystem comparison tool capabilities, graph generation, and
various export formats available. Performance tuning guidance is included
for long-duration testing, resource optimization, and result validation.

Troubleshooting sections address common issues with filesystem creation,
mount failures, missing results, and comparison analysis problems. Best
practices are provided for configuration, testing methodology, analysis
approaches, and CI/CD integration.

Example workflows demonstrate practical usage patterns including
development workflows with iterative testing, kernel patch testing with
A/B comparisons, and filesystem optimization workflows for finding optimal
configurations.

Advanced topics cover custom filesystem configuration addition,
integration with other kdevops workflows, and performance regression
detection setup.

Documentation includes comprehensive examples of using declared hosts with
fio-tests for bare metal testing, production hardware validation, kernel
regression testing, and multi-filesystem comparisons. Prerequisites,
troubleshooting, and example workflows are provided for effective use of
declared hosts with filesystem testing.

The main fio-tests.md documentation is updated with a cross-reference to
the new filesystem testing documentation, providing users with a clear
entry point to discover these capabilities.

Generated-by: Claude AI
Signed-off-by: Luis Chamberlain <[email protected]>

Author: mcgrof