Fix TestPlan validation and recognition default TestPlan of Scheme

[Ernest0-Production]

Resolves #6918

Short description 📝

• Adds logic with the definition of the default TestPlan that is set in Scheme.
• Corrects the projectPath mapping of XCTestPlan, building the path relative to the manifest directory, not the root.
• Corrects and structures the validation of the scheme to meet at least one of the conditions:

1. the presence of the passed test-plan in the list of test plans of the scheme
2. the presence of the default testplan
3. at least one test target

• The code has been linted using run mise run lint-fix
• The change is tested via unit testing or acceptance testing, or both
• The title of the PR is formulated in a way that is usable as a changelog entry
• In case the PR introduces changes that affect users, the documentation has been updated

Reviewer checklist ✅

• The code architecture and patterns are consistent with the rest of the codebase
• Reviewer has checked that, if needed, the PR includes the label changelog:added, changelog:fixed, or changelog:changed, and the title is usable as a changelog entry

[tuist]

🛠️ Tuist Run Report 🛠️

Tuist Tests 🧪

Command	Status	Cache hit rate	Tests	Skipped	Ran	Commit
test TuistAutomationAcceptanceTests	✅	90 %	1	0	1	39ffe9390
test TuistDependenciesAcceptanceTests	✅	90 %	1	0	1	b2dad7691
test TuistGeneratorAcceptanceTests	✅	90 %	1	0	1	ce2a48090
test TuistKitAcceptanceTests	✅	90 %	1	0	1	d3637e205
test TuistUnitTests	❌	90 %	23	0	23	6f71e255d

[Ernest0-Production]

Damn, looks like I'll have to fix a lot of unit tests in TestServiceTests.swift... :C

In many tests, the mocked graph contains a diagram with a reference to a test target whose projectPath is missing from the graph.
This is an initially incorrect state of the project even for a mock.