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.

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