Include stack trace in failed test info

Nobody has claimed this yet.

I opened an issue on the Gleam repository about a potential inconsistency in how let assert failures are reported ([issue 4703](https://github.com/gleam-lang/gleam/issues/4703#issuecomment-2990200796)). The behavior differs between gleam run and gleam test.

Here’s an example of the output from `gleam test`:

```
test/phone_number_test.gleam
test: cleans_the_number_test
error: Pattern match failed
site: phone_number:9
value: ["2", "2", "3", "4", "5", "6", "7", "8", "9", "0"]
```

And from `gleam run`:

```
runtime error: let assert

Pattern match failed, no pattern matched the value.

unmatched value:
["0", "0", "0", "0", "0", "0", "0", "0", "0", "0"]

stacktrace:
phone_number.-clean/1-fun-0- src\phone_number.gleam:9
```

As I noted in the issue:

> “It seems like `gleam run` is much more explicit—it clearly signals that the developer made a wrong assertion, with a red-colored title and a full stack trace (e.g., src\phone_number.gleam:9). On the other hand, `gleam test` reports the failure more subtly, almost as if everything ran normally.”

Wouldn’t it be a good idea for `gleam test` to adopt a stricter and more visible approach to `let assert` failures—similar to `gleam run`—since these are developer errors that benefit from being highlighted clearly?

Reproduce the difference using gleam test and gleam run with test/phone_number_test.gleam and src/phone_number.gleam, focusing on the let assert failure at phone_number:9. Trace how each command formats failures and compare the existing test output with the run output; done means failed test information includes a clear stack trace and the relevant tests pass.

Written by the indexing model from the issue text.