In certain circumstances, `go env GOPATH` can return 'NUL' or
'/dev/null'. This updates goModules to handle that case.

This updates package.json and adds the relevant propsed API definitions
to prepare for implementing the test API. This commit should be reverted
once the test API is no longer experimental.

This takes a dynamic approach to test discovery. Tree nodes will be
populated as they are expanded in the UI. Tests in open files will be
added.

Fixes golang#1579

This fixes discovery of workspace tests when loading the workspace, when
adding and removing workspace folders, and when adding and removing
files.

This adds the capability to the test provider to actually run tests.
Debugging is still not supported.

- Log messages with 'file.go:line:' are attached to the correct location
- Output without file information is only reported via the test console
- Benchmark lifecycle is handled better

- Add test_events.md to detail what go test -json looks like
- Add comments
- Also, address gerrit comments

Change function names and add comments.

Fix a dumb mistake and remove a TODO.