src/goTestExplorer: implement a test provider for the new test api

[firelizzard18]

What this does:

• Implements a test provider using the new API
• Populates the test explorer with a list of modules and/or workspaces
• Populates each module/workspace/package/file as it is expanded
• Creates test entries for the current file, and package/module parents
• Runs tests!

What this does not:

• Debug tests
• Handle stretchr suites

Issues:

• Handling of benchmarks isn't great. But I'm not sure it can get much
  better without changes to go test.
• If a test has subtests, I add those subtests. This can get annoying if
  you have a bunch of subtests. Should this be configurable? Disabling
  testing.followRunningTest can improve this UX.

Fixes #1579

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[firelizzard18]

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