Slow test reporter

[hadley]

Initial prototype by claude, but lots of iteration by me.

Fixes #1466

[hadley]

@gaborcsardi would you mind taking a look at this one? It'd be useful if you'd try running it on a couple of projects because it's hard to test the output.

[gaborcsardi]

I like it!

── Slowest tests: ───────────────────────────────────────────────────────────────────────────────────────
[4.19s] test-watcher.R: watcher works correctly
[1.73s] test-parallel-crash.R: crash
[1.17s] test-parallel-errors.R: error in parallel setup code
[0.86s] test-parallel.R: snapshots
[0.83s] test-parallel.R: snapshots are removed if test file has no snapshots
[0.82s] test-parallel.R: new snapshots are added
[0.76s] test-parallel.R: ok
[0.75s] test-parallel.R: snapshots are removed if test file is removed
[0.73s] test-parallel-outside.R: error outside of test_that()
[0.68s] test-parallel.R: fail
... and 482 more slow tests

I understand where this is coming from, but to see that that I have 482 (!) more slow tests is kind of demoralizing....