tests/doc: add template for documenting test rationale and design

Signed-off-by: Adrian-Stanea <Adrian.Stanea@analog.com>

Author: Adrian-Stanea

tests/doc/README.md

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+# Test Design Documentation
+
+This directory contains design documentation for tests in the libm2k test suite. Each document explains the rationale behind test design decisions, making it easier to understand, maintain, and extend the test suite.
+
+## Purpose
+
+- **Document design decisions** - Explain why tests are structured the way they are
+- **Capture domain knowledge** - Record hardware-specific considerations
+- **Enable maintenance** - Help future developers understand test intent
+- **Track issue references** - Link tests to the bugs/features they validate
+
+## Document Structure
+
+Each test document should follow this template:
+
+```markdown
+# Test: [Test Name]
+
+## Overview
+| Field                | Value                                |
+| -------------------- | ------------------------------------ |
+| **Test Name**        | `test_function_name`                 |
+| **Test File**        | `tests/file.py`                      |
+| **Related Issue**    | Link to GitHub issue (if applicable) |
+| **Minimum Firmware** | Version requirement (if applicable)  |
+
+## Problem Statement
+What problem or feature does this test validate?
+
+## Test Configuration
+What parameters are used and why?
+
+## Design Rationale
+Why was this approach chosen? What alternatives were considered?
+
+## Expected Results
+What indicates pass/fail? What error messages mean what?
+
+## Running the Test
+Command-line examples for running the test.
+
+## Running the Test
+Instructions for running the test.
+
+## References
+Links to issues, documentation, datasheets, etc.
+```
+
+## Naming Convention
+
+Documentation files should be named after the test method they document:
+- `test_dual_channel_waveform_sync.md`
+
+## Index
+
+| Document                                                                 | Test                              | Description                                  |
+| ------------------------------------------------------------------------ | --------------------------------- | -------------------------------------------- |
+| [test_dual_channel_waveform_sync.md](test_dual_channel_waveform_sync.md) | `test_dual_channel_waveform_sync` | Validates dual-channel phase synchronization |
+