Add a test showing RBS signatures are being deleted by the deadcode remover

[Morriar]

@shioyama it looks like it works. Do you have an example where it didn't?

[KaanOzkan]

I think we can close since we have

spoom/test/spoom/deadcode/remover_test.rb

Lines 965 to 988 in 747c18a

	def test_removes_node_with_comments
	res = remove(<<~RB, "bar")
	class Foo
	def foo; end
	# Some comments
	# related
	# to bar
	def bar
	something
	end
	def baz; end
	end
	RB
	assert_equal(<<~RB, res)
	class Foo
	def foo; end
	def baz; end
	end
	RB
	end

[Morriar]

Very stale