SOLR-17654: DistribFileStore._getRealPath() has issues on Windows

[mlbiscoc]

https://issues.apache.org/jira/browse/SOLR-17654

Description

Number of tests failing on windows due to beginning slash in a path not being stripped to become relative.

Solution

Revert to old logic but use more modern FileSystems.getDefault().getSeparator() instead.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[dsmiley]

(thinking out loud)
I'd love to get a JVM running in Windows in a container maybe (not possible with macOS host?) or I suppose VirtualBox if I have to. Actually, I could probably use AWS free tier and use an AMI with Java to do some temporary tinkering. Or maybe someone recommends another option.

[janhoy]

(thinking out loud) I'd love to get a JVM running in Windows in a container maybe (not possible with macOS host?) or I suppose VirtualBox if I have to. Actually, I could probably use AWS free tier and use an AMI with Java to do some temporary tinkering. Or maybe someone recommends another option.

I use UTM (https://mac.getutm.app) on M1 mac to spin up Win11 ARM edition. You can share your disk and test stuff. PS: Wonder if we could tell GithubActions to run tests on windows runner for a PR? Perhaps triggered by some file touched?

[dsmiley]

Thanks Jan for that suggestion! I grabbed that and intend to play with it this weekend.
I'll merge this PR shortly.