Pass file directory as cwd for Run test of CodeLens

[Gowee]

In the previous implementation:

When there is a Cargo Workspace including several member packages, clicking CodeLens Run test button in source code files of a member package triggers shell execution cargo test -- --no-capture ... without specifying a current working directory.It appears that the default cwd is the Workspace / top-level package root (as expected).

Consequently, no targeted tests are actually perfomed as cargo test does not cover member packages unless --workspace is specified.

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To fix the problem, there are two feasible ways:

1. Adding --workspace option to cargo test.
2. Setting the cwd properly.

The first way is much slower than the second as it takes more time to filter out unnecessary tests outside current member packages.

So this commit passes the file directory as Cmd.cwd in CodeLens Command to rls-vscode so that cargo test ... can be executed with proper cwd.

[bors]

☔ The latest upstream changes (presumably #1740) made this pull request unmergeable. Please resolve the merge conflicts.