Tests table: auto expand/don't group if group has only one item.

The tests table in `/test///` groups tests by their path. However, in many cases paths have only one test or subpath associated with them. I suggest ignoring grouping if it groups only one item, to ease navigation.

With this change, a tree that looks like this:
```
.
├── A
│ └── p
├── B
│ └── C
│ ├── q
│ ├── r
│ └── s
└── D
│ └── E
│ └── t
└── F
└── G
├── H
│ └── u
└── I
└── v
```

Would instead look like this:
```
.
├── A.p # single test in a group
├── B.C # single group in a group
│ ├── q
│ ├── r
│ └── s
├── D.E.t # multiple levels compacted
└── F.G
├── H.u
└── I.v
```

Start by tracing the tests table implementation for the /test/// route and how it builds grouped paths. Verify the displayed tree against the issue's before-and-after examples; done means groups containing one item are compacted while multi-item groups remain expanded.

Written by the indexing model from the issue text.