Depends on #6303
## Summary
Extract shared test boilerplate (includes, type aliases, test fixture
macros) from 47 `test_gemm_quant_*` files into a single
`test_gemm_quant_common.hpp` header. Each test file is reduced from ~50
lines of boilerplate to ~5 lines.
| Metric | Value |
|--------|-------|
| Files changed | 48 |
| Insertions | +413 |
| Deletions | −1,106 |
| **Net lines removed** | **−693** |
### What changed
| Before | After |
|--------|-------|
| 47 test files, each with ~50 lines of identical includes, type
aliases, and fixture macros | 1 shared header
(`test_gemm_quant_common.hpp`) + 47 thin files (~5 lines each: include +
params) |
### Readability assessment
A code realist review confirmed this change **improves readability**:
the 47 test files had identical boilerplate obscuring the only
meaningful content — the `GemmConfig` type alias and test dimensions.
After the refactoring, each file's unique configuration is immediately
visible, and adding a new test variant requires specifying only the
varying parameters instead of copying 50 lines.
### Cumulative cleanup series stats
| PR | Description | Net lines |
|----|-------------|-----------|
| #6300 | Remove 61 dead `#if 0` blocks | −2,648 |
| #6302 | Remove 41 commented-out dead code blocks | −2,861 |
| #6303 | Remove 4 orphaned files | −3,886 |
| This PR | Extract gemm_quant test boilerplate | −693 |
| **Total** | | **−10,088** |
