Add license to test files with REUSE.toml file

Perhaps should license test files. Put in testsuite/REUSE.toml:

```toml
version = ???

[[annotations]]
path = "**"                                                                                   
precedence = "aggregate"
SPDX-FileCopyrightText = "2026 srcML, LLC. (www.srcML.org)"
SPDX-License-Identifier = "GPL-3.0-only"
```

Not sure of the details yet.

