Auto merge of #118548 - Enselic:bench-padding, r=thomcc,ChrisDenton

libtest: Fix padding of benchmarks run as tests

### Summary

The first commit adds regression tests for libtest padding.

The second commit fixes padding for benches run as tests and updates the blessed output of the regression tests to make it clear what effect the fix has on padding.

Closes #104092 which is **E-help-wanted** and **regression-from-stable-to-stable**

### More details

Before this fix we applied padding _before_ manually doing what `convert_benchmarks_to_tests()` does which affects padding calculations. Instead use `convert_benchmarks_to_tests()` first if applicable and then apply padding afterwards so it becomes correct.

Benches should only be padded when run as benches to make it easy to compare the benchmark numbers. Not when run as tests.

r? `@ghost` until CI passes.

Author: bors