fix: prevent duplicate sway unit test execution (FuelLabs#3986)

## Description
This PR adds a step to execution of unit tests so that duplicate unit
tests are omitted. Duplicate tests are generated from dependencies
declared with `dep` keyword in a duplicate manner (see FuelLabs#3947). Until
that is fixed this PR de-duplicates sway unit tests based on their
`span`. Once the fix for FuelLabs#3947 is ready we can remove the changes from
this PR. This is also commented in the code.

closes FuelLabs#3813.

## 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).
- [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.

forc-test/src/lib.rs

@@ -1,3 +1,4 @@
+use std::collections::HashSet;
 use std::{collections::HashMap, fs, path::PathBuf, sync::Arc};
 
 use forc_pkg as pkg;
@@ -189,12 +190,15 @@ impl<'a> PackageTests {
 
     /// Run all tests for this package and collect their results.
     pub(crate) fn run_tests(&self) -> anyhow::Result<TestedPackage> {
+        // TODO: Remove this once https://github.com/FuelLabs/sway/issues/3947 is solved.
+        let mut visited_tests = HashSet::new();
         let pkg_with_tests = self.built_pkg_with_tests();
         // TODO: We can easily parallelise this, but let's wait until testing is stable first.
         let tests = pkg_with_tests
             .entries
             .iter()
             .filter_map(|entry| entry.kind.test().map(|test| (entry, test)))
+            .filter(|(_, test_entry)| visited_tests.insert(&test_entry.span))
             .map(|(entry, test_entry)| {
                 let offset = u32::try_from(entry.finalized.imm)
                     .expect("test instruction offset out of range");