Move debugger_visualizer tests to separate crate

[lucacasonato]

To prevent having to raise MSRV again, we move the debugger_visualizer to a separate crate. This crate is only compiled when the feature debugger_visualizer is enabled, because only then the debugger_visualizer test is added as a compile target.

Implements @valenting's suggestion from #849

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 :warning:

Comparison is base (0e25146) 82.44% compared to head (4b31c13) 82.43%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #853 +/- ## ========================================== - Coverage 82.44% 82.43% -0.01% ========================================== Files 20 20 Lines 3343 3348 +5 ========================================== + Hits 2756 2760 +4 - Misses 587 588 +1 ``` | [Impacted Files](https://app.codecov.io/gh/servo/rust-url/pull/853?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo) | Coverage Δ | | |---|---|---| | [url/src/lib.rs](https://app.codecov.io/gh/servo/rust-url/pull/853?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo#diff-dXJsL3NyYy9saWIucnM=) | `75.96% <ø> (-0.11%)` | :arrow_down: | ... and [4 files with indirect coverage changes](https://app.codecov.io/gh/servo/rust-url/pull/853/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=servo)

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[valenting]

Sweet. Thank you for implementing this.