added regtest for XDistances

Description

We added a retest for XDistances

Type of contribution

[ ] changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
[ X] changes to a module not authored by you
[ ] new module contribution or edit of a module authored by you

Copyright

[X ] I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

[ ] the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

[X ] I added a new regtest or modified an existing regtest to validate my changes.
[ ] I verified that all regtests are passed successfully on GitHub Actions.

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 :tada:

Comparison is base (27e06a7) 85.12% compared to head (d9eff37) 85.15%.

:exclamation: Current head d9eff37 differs from pull request most recent head c72b1a6. Consider uploading reports for the commit c72b1a6 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #964 +/- ## ========================================== + Coverage 85.12% 85.15% +0.03% ========================================== Files 600 600 Lines 55555 55555 ========================================== + Hits 47292 47310 +18 + Misses 8263 8245 -18 ``` | [Impacted Files](https://app.codecov.io/gh/plumed/plumed2/pull/964?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=plumed) | Coverage Δ | | |---|---|---| | [src/tools/OpenMP.cpp](https://app.codecov.io/gh/plumed/plumed2/pull/964?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=plumed#diff-c3JjL3Rvb2xzL09wZW5NUC5jcHA=) | `100.00% <100.00%> (+12.50%)` | :arrow_up: | | [src/tools/OpenMP.h](https://app.codecov.io/gh/plumed/plumed2/pull/964?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=plumed#diff-c3JjL3Rvb2xzL09wZW5NUC5o) | `100.00% <100.00%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/plumed/plumed2/pull/964/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=plumed)

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plumed / plumed2