Fail nicer on missing expected_result in test.toml files (#5941)

## Description

If the `test.toml` file in a test does not contain the entries
`expected_result` and `expected_result_new_encoding`, the end-to-end
tests fail with an unhelpful error message.

This PR improves the error message.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

test/src/e2e_vm_tests/mod.rs

@@ -293,7 +293,7 @@ impl TestContext {
 
         match category {
             TestCategory::Runs => {
-                let expected_result = expected_result.unwrap();
+                let expected_result = expected_result.expect("No expected result found. This is likely because test.toml is missing either an \"expected_result_new_encoding\" or \"expected_result\" entry");
 
                 let (result, out) =
                     run_and_capture_output(|| harness::compile_to_bytes(&name, &run_config)).await;