[data-shapes] Namespace of `ex:` in tests (#276)

ajnelson-nist has just created a new issue for https://github.com/w3c/data-shapes:

== Namespace of `ex:` in tests ==
I just ran `grep -R '@prefix ex:'` and noticed that the test IRIs are either similar to this...

```
./tests/sparql/pre-binding/unsupported-sparql-001.ttl:@prefix ex: <http://datashapes.org/sh/tests/sparql/pre-binding/unsupported-sparql-001.test#> .
```

... or this:

```
./tests/core/node/xone-duplicate-data.ttl:@prefix ex: <http://example.org/shacl-test/> .
```

Should these be unified, in domain and path style?  The former scopes tests' IRIs nicely, but the latter avoids introducing a domain outside of W3C.

Please view or discuss this issue at https://github.com/w3c/data-shapes/issues/276 using your GitHub account


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