Inconsistent test subject URLs in JSON report

Hi,

The files in an EPUB are (truncated):
```
mimetype
META-INF/container.xml
OPS/nav.xhtml
OPS/package.opf
OPS/xhtml/Sources.html
...
```
Those same files, with this or that failing issue, are reported as follows (truncated):
```json
"assertions": [
{
"@type": "earl:assertion",
...
"earl:testSubject": {
"url": "/OPS/nav.xhtml"
}
},
{
"@type": "earl:assertion",
...
"earl:testSubject": {
"url": "OPS/package.opf",
}
},
{
"@type": "earl:assertion",
...
"earl:testSubject": {
"url": "xhtml/Sources.html",
}
},
{
"@type": "earl:assertion",
...
"earl:testSubject": {
"url": "nav.xhtml",
}
},
...
]
```
Note how the `"url"` path sometimes does, sometimes does not contain the `OPS/` root folder. This makes tooling and automation a bit difficult, and should be fixed.

Cheers.

Start by tracing the JSON report generation for EPUB entries and compare how the paths in earl:testSubject.url are constructed for nav.xhtml, package.opf, and xhtml/Sources.html. Done means every reported URL uses one consistent path-root convention, so tooling can process the report without special cases.

Written by the indexing model from the issue text.