debug: find a way to use dlv-dap with `debug test` codelens

[hyangah]

Due to the issue #855
we currently don't have an easy way to run debugging using dlv-dap.
Either we need to fix the issue, or take advantage of "go.delveConfig" setting.

cc @polinasok @suzmue

[gopherbot]

Change https://golang.org/cl/306969 mentions this issue: src/goTest.ts: allow users to select dlv-dap in codelenses

[hyangah]

@suzmue I think we need documentation update.

[gopherbot]

Change https://golang.org/cl/309749 mentions this issue: docs/dlv-dap.md: include go.delveConfig setting for debugAdapter