API for grouping tests, a la `describe`

Nobody has claimed this yet.

Something I often do when using Mocha:

```js
describe('my-library', () => {
describe('method_one', () => {
it('returns a string', () => {
assert.equal(typeof my_library.method_one(), 'string');
});

// ...
});

describe('method_two', () => {
it('returns a number', () => {
assert.equal(typeof my_library.method_two(), 'number');
});

// ...
});
});
```

This results in neatly indented output...

```
my-library
method_one
✓ returns a string
✓ ...
method_two
✓ returns a number
✓ ...
```

...and makes it easier to identify (and isolate/skip) problematic groups of tests.

It'd be nice to have something equivalent in uvu — perhaps something like this?

```js
test.group('my-library', () => {
test.group('method_one', () => {
test('returns a string', () => {
assert.equal(typeof my_library.method_one(), 'string');
});

// ...
});

test.group('method_two', () => {
test('returns a number', () => {
assert.equal(typeof my_library.method_two(), 'number');
});

// ...
});
});
```

Plus `test.group.only` etc. Thoughts?

Start with the issue's nested test.group examples and the described indented output; the payload names no files or tests. Define how nested groups and test.group.only-style isolation should behave, then verify grouped tests render hierarchically and can be isolated or skipped.

Written by the indexing model from the issue text.