Fix: Allow Nextest to Work with Manifests Outside of Workspace Root Path (nextest-rs/nextest issue #1684) v2

[PegasusPlusUS]

Resubmitting for previous one polluted by huge background file commits in GitHub codespace.

Fix: Allow Nextest to Work with Manifests Outside of Workspace Root Path
Description
This pull request addresses an issue where Nextest fails to function correctly when the Cargo.toml manifest file is located outside of the workspace root path. The changes ensure that Nextest can handle such scenarios gracefully.

Changes
Updated the guppy crate to use pathdiff for relative path of manifest from workspace root, no matter it is outside of the workspace root.

Modified guppy/src/graph/build.rs fn workspace_path() #402~#408 , add 3 test cases on relative path result for unix and windodws.
Remove debug assert for relative path in fn convert_forward_slashes() #990-#994 so that if manifest path may be on another driver on windows.

Issue
This pull request fixes nextest-rs/nextest issue #1684.

Testing
Verified that Nextest can now correctly locate and use Cargo.toml files outside the workspace root.

Ran existing test suites to ensure no regressions were introduced.

[PegasusPlusUS]

Before patch, cargo t can work, cargo nextest run can't work:

Using patched guppy, now cargo nextest run can test normally:

workspace on C:, manifest on D:

[PegasusPlusUS]

Don't know why there still has background commit that contained in PR, which I can't see these files before submitting PR. Hope this time these small amout of files (compared to previously huge amount of files) not affecting review and merge process.

[PegasusPlusUS]

Updated, test all passed on Ubuntu and Windows, sample workspace on Ubuntu and Windows, and extra workspace that cross drives on Windows all passed.

[PegasusPlusUS]

Since convert_forward_slashes() will convert relative path and clone absolute path,
these 6 lines of code can be done as 1 line:

Self::Path(convert_relative_forward_slashes(path_diff).into_boxed_path())