Improve desc_filter so you can run single BDD-style tests (aka run just a single it test from describe)

[kubajal]

Currently, desc_filter let's you run either a single test_that or a single describe suite of tests. This PR adds support for selecting just a single it(...) test from a describe suite by adding the &&& separator to the desc argument.
So e.g. running testthat::test_file("tests.R", desc="level 0&&&level 1 A")

# tests.R
describe("level 0", {
  it("level 1 A", {}),
  it("level 1 B", {})
})

effectively triggers just

describe("level 0", {
  it("level 1 A", {}),
})

This PR is a continuation of #1904 and #1840.

[kubajal]

The error on Ubuntu is not related to my change and has been observed in other PRs too, e.g. #2061:

Error: Error: running the tests in ‘testthat.R’ failed
Error: Error: Failure in `/home/runner/work/_temp/package/testthat/testthat-tests/testthat.Rout.fail`
r ('test-local.R:38:3'): can override translation of error messages ─────
<notSubsettableError/error/condition>
Error in `mean[[1]]`: object of type 'closure' is not subsettable
Backtrace:

[kubajal]

@hadley could you have a look? This PR is ready for review.

[hadley]

@kubajal I'll take a look when I'm next working on testthat.