x/tools/gopls: integrate test discovery

[firelizzard18]

I would like to migrate the test discovery services of vscode-go into gopls so they can be implemented in Go and utilize the rich type information provided by the go/* packages instead of the current JavaScript implementation which relies on recursive directory walks and analyzing document symbols. Moving test discovery into gopls should improve performance and would bake it far easier to implement improvements such as detecting subtests in cases where that can be done deterministically.

[firelizzard18]

I meant to open this on the Go repository golang/go#59445