E2E tests

[marinofelipe]

Introduce E2E tests that run spmgraph commands via Process.

TODO

• Update lint test, once the other PRs are merged

[Copilot]

Pull Request Overview

Introduces comprehensive E2E tests for the spmgraph command-line tool that validate core functionality by running actual processes. The PR adds a dedicated test plan configuration, fixture package for testing, and comprehensive test coverage for all major commands.

Key changes:

• Adds E2E test suite that runs spmgraph commands via Process to test real-world usage
• Creates fixture package structure with targets and dependencies for testing
• Implements test plan configuration with Address Sanitizer enabled for graphviz compatibility

Reviewed Changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
spmgraph.xctestplan	Test plan configuration with environment variables and command-line arguments for E2E testing
Tests/SPMGraphExecutableTests/SPMGraphExecutableE2ETests.swift	Comprehensive E2E test suite covering all major spmgraph commands
Tests/Fixtures/Package/*	Test fixture package with targets, dependencies, and test files
Sources/SPMGraphVisualize/SPMGraphVisualize.swift	Updates to support AbsolutePath for output file handling
Sources/Core/Extensions/ProcessInfo+Core.swift	Helper extension to detect test environment
Package.swift	Adds SPMGraphExecutableTests target
.github/workflows/pull-request.yml	Updates CI to use xcodebuild with test execution

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