Remove requirement to not have "/" in test names? (Junit 5)

Fantastic tool, thank you! I only hit a little snag: in my project I already have many Kotest/Junit5 units named after HTTP endpoints such as `GET /example`, and I'm hitting this error:

```
Caused by: java.lang.IllegalStateException: Test name cannot contain '/', was ...
at com.diffplug.selfie.junit5.SnapshotFileProgress.startTest(SnapshotSystemJUnit5.kt:188)
```

This happens for every test even though they do not invoke Selfie for snapshots. Could this requirement be loosened somehow? For example:

* Escape the resulting name of snapshot files to remove the slash
* Do not run this check in tests that do not make use of Selfie

Thank you.

Start at SnapshotSystemJUnit5.kt:188, where SnapshotFileProgress.startTest rejects test names containing '/'. Reproduce the issue with a JUnit 5 test named like `GET /example`, then determine whether the check should be relaxed or limited to tests using Selfie. Done means such test names no longer fail unnecessarily while snapshot file names remain valid.

Scritto dal modello di indicizzazione a partire dal testo della issue.