frint-router-react: improve test coverage

Current coverage report:

```
$ (cd ./packages/frint-router-react && npm run cover:report)

----------------|----------|----------|----------|----------|----------------|
File | % Stmts | % Branch | % Funcs | % Lines |Uncovered Lines |
----------------|----------|----------|----------|----------|----------------|
All files | 94.95 | 86.76 | 81.48 | 97.89 | |
Link.js | 100 | 85.71 | 100 | 100 | 18,42,93 |
Link.spec.js | 97.6 | 50 | 91.67 | 97.58 | 18,268,282 |
Route.js | 100 | 96.55 | 100 | 100 | 20 |
Route.spec.js | 96.53 | 100 | 86.84 | 100 | |
Switch.js | 100 | 80 | 100 | 100 | 15,37 |
Switch.spec.js | 86.82 | 66.67 | 65.79 | 94.07 |... 65,67,75,77 |
----------------|----------|----------|----------|----------|----------------|
```

Run `(cd ./packages/frint-router-react && npm run cover:report)` and inspect the uncovered lines in Link.js, Route.js, and Switch.js alongside their corresponding spec files. Add tests for the reported gaps and rerun the coverage report to verify that coverage improves.

Written by the indexing model from the issue text.