`lib.debug.runTests` provides a unit test evaluator for Nix, but its
results are returned in a raw and difficult-to-read form.
Currently, different callers output the results in various ways:
`builtins.throw (builtins.toJSON failures)` and `builtins.throw ("Tests
failed: " + lib.generators.toPretty { } failures)` are both used.
This change adds a new `lib.debug.throwTestFailures` function which
displays the results nicely before throwing an exception (or returns
`null` if no failures are given), unifying these disparate call-sites.
First, each failing test is pretty-printed in a `trace` message:
```
trace: FAIL testDerivation:
Expected: <derivation a>
Result: <derivation b>
```
Then, an exception is thrown containing the number of tests that failed
(and their names), followed by the raw JSON of the results (for parity
with previous usage, and because `lib.generators.toPretty` sometimes
omits information that `builins.toJSON` includes):
```
error:
… while evaluating the file '...':
… caused by explicit throw
at /nix/store/.../lib/debug.nix:528:7:
527| in
528| throw (
| ^
529| builtins.seq traceFailures (
error: 1 tests failed:
- testDerivation
[{"expected":"/nix/store/xh7kyqp69mxkwspmi81a94m9xx74r8dr-a","name":"testDerivation","result":"/nix/store/503l84nir4zw57d1shfhai25bxxn16c6-b"}]
```
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