Fix discovery of test methods in specs

[thomasmarshall]

Motivation

Closes #3722.

Methods starting test_ are picked up my Minitest but were not being discovered by the LSP.

describe "MySpec" do
  def test_foo
    assert true
  end
end

Implementation

This commit adds a #on_def_node_enter handler to register test items for methods starting test_ in describe blocks. It should not match non-test methods (e.g. helper methods), nor should it match methods named test_ in non-spec blocks.

Automated Tests

I have added a new testfor this fix.

Manual Tests

1. Add a spec with a test_ method
2. See that it is discovered by the LSP

Before	After