Align PR summary table to be more readable in its raw form

[Kobzol]

Aligns the columns to the same width (more or less), to be more readable in plain text. Let me know what you think before I update all the tests :)

Before:

| | Regressions 😿 <br />(primary) | Regressions 😿 <br />(secondary) | Improvements 🎉 <br />(primary) | Improvements 🎉 <br />(secondary) | All 😿 🎉 <br />(primary) |
|:---:|:---:|:---:|:---:|:---:|:---:|
| count[^1] | 2 | 0 | 2 | 0 | 4 |
| mean[^2] | 150.0% | N/A | -62.5% | N/A | 43.8% |
| max | 200.0% | N/A | -75.0% | N/A | 200.0% |

[^1]: *number of relevant changes*
[^2]: *the arithmetic mean of the percent change*

After:

|            | Regressions 😿 <br />(primary) | Regressions 😿 <br />(secondary) | Improvements 🎉 <br />(primary) | Improvements 🎉 <br />(secondary) | All 😿 🎉 <br />(primary) |
|:----------:|:------------------------------:|:--------------------------------:|:-------------------------------:|:---------------------------------:|:------------------------:|
| count[^1]  | 2                              | 0                                | 2                               | 0                                 | 4                        |
| mean[^2]   | 150.0%                         | N/A                              | -62.5%                          | N/A                               | 43.8%                    |
| max        | 200.0%                         | N/A                              | -75.0%                          | N/A                               | 200.0%                   |

[^1]: *number of relevant changes*
[^2]: *the arithmetic mean of the percent change*

Fixes: #1310

[klensy]

Maybe remove emoji from column titles?

When there was not a table (in previous version of reports), but lines with emoji, was used only a few ones when appropriate, but now there a lot of it unconditionally.

[Kobzol]

I realize that emojis are mostly useless for plain text, but at least for me it's quite useful in GitHub comments, as it allows me to quickly identify which columns are the regressions and which are the improvements.

[rylev]

Looks good. Just need to fix the CI breakage.

[Kobzol]

Fixed tests and changed the assert to debug_assert.