testing: expand integration testing with gopls enabled

[hyangah]

Workspace setup:

• open a folder
• open a file
• open a multi-folder workspace

Focus on modules mode (since Go is now moving away from GOPATH)

Features: (see https://github.com/golang/tools/tree/master/gopls/internal/regtest)

• codelens
• completion
• configuration
• go to definition
• references
• diagnostics
• documentation
• imports
• tags
• go.mod file related features...(?)

[gopherbot]

Change https://golang.org/cl/266418 mentions this issue: test/gopls: change test environment setup to use single file edit