Account for nested `describe()` in R test explorer

The positron-r test explorer still has some missing functionality around nested describe() like this:

describe("top-level describe with describe inside", {
  describe("nested describe 1", {
    it("can 'add' two numbers", {
      expect_equal(1 + 1, addition(1, 1))
      #expect_equal('a', 'b')
    })
  })
  describe("nested describe 2", {
    it("can multiply two numbers", {
      expect_equal(10 * 2, multiplication(10, 2))
    })
    it("can handle division by 0") #not yet implemented
  })
})

Problems include:

• describe("top-level describe with describe inside", { ... }) does not appear in the tree view, because we only walk up to the immediate parent of an it() during test discovery.
• If the user clicks on ▶️ for a nested describe(), it would be nice to hop up to the enclosing, top-level describe() and run that instead of doing nothing (other than logging that this is impossible). Ditto for an it().
• Clicking on ▶️ in the tree view for a describe() with exactly 1 (one) it() results in running the it(), which obviously fails. Why does it run the it()? Need to determine if this is a "me" problem or a bug in VS Code. Clicking on ▶️ in the gutter of the test file does run the single describe() appropriately. This has nothing to do with a nested describe() btw.

Start in the positron-r test explorer's discovery and tree execution paths, using the nested describe() example in the issue. Compare tree-view ▶️ behavior with running from the test file gutter in VS Code; done means nested describes appear, their actions run the enclosing top-level describe, and the single-it case is correctly handled.