x/tools/gopls: migrate "test generation" features to gopls

[n1lesh]

vscode-go uses the gotests package to generate unit tests, which makes it hard to adjust the test generation behavior as mentioned here: vscode-go issue #1594.

We could move the test generation logic to gopls, which provides a few benefits,

1. Flexibility in modifying logic around test generation. Current vscode-go implementation uses gotests as a child process to generate tests, which makes it hard to customize test generation.
2. Reduction in the number of 3rd party tools for vscode-go. gotests package moves within gopls.

More about it here.

/cc @stamblerre

[beoran]

Currently, gotests is written to work as as a command only. I reccommend you would work together with @cweil and help to develop a version of gotests that also can be used as a module library, and which has the additional features you would like. This will then also benefit people who are not using gopls or VisualStudio, and want to integrate this in a different editor, like myself.