gopls/internal: add coverage command to compute test coverage
gopls/internal: add coverage command to compute test coverage

Running gopls/internal/coverage/coverage.go in the tools directory
produces a coverage file (/tmp/cover.out by default) and a report
showing how well the gopls tests cover the packages in internal/lsp.

Change-Id: I1a7a22321f807ae54194833ee6a8e2a80bd9dca0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/303290
Run-TryBot: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
Trust: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools