Add solo_test, solo_group API

[srawlins]

Fixes #127

This is a fairly simple implementation of solo_test and solo_group, restricted to the "backend" code (lib/src/backend/declarer.dart and lib/src/backend/metadata.dart).

WIP

This PR is functional, but I'm throwing it out there as a "request for comments." What do the authors think?

TODO:

• Write up some text in README.
• Write up some doc comments for solo_test and solo_group. These should be lengthy, with explanations of inheritance, and what to expect in output.

Notes

• The real API change is to add a solo: named arg to test and group, but the backward compatibility, and real beauty, comes in implementing the package:unittest methods: solo_test and solo_group. Those five characters, solo_ are what people love so much about unittest's feature.

• The feature works by declaring all non-solo tests or groups as "skipped". So there is no new status in the reporting. For example:

  $ cat a.dart
  import 'lib/test.dart';
  
  void main() {
    test('foo', () { expect('a', equals('a')); });
    test('bar', () { expect('a', equals('a')); });
    solo_test('baz', () { expect('a', equals('a')); });
  }
  $ dart a.dart
  00:00 +0: foo
  00:00 +0 ~1: bar
  00:00 +0 ~2: baz
  00:00 +1 ~2: All tests passed!
  

  We see that it looks like one test ran, and two were skipped. This is the analogy of "solo".

• I don't think this feature needs a new status (something like "00:00 !1 ~2: All tests passed!"), but we could if we wanted.

• We could change the "All tests passed!" text to be different if any tests were skipped, but that would be a separate feature, unrelated to this PR.

• I did not change the return code if a solo test/group is found. We could if we wanted to, but again, this could be a separate feature (different return code if any tests are skipped), unrelated to this PR.

• We could add one line of text to the output, like "Solo test/group found! Skipping everything not solo." But I don't think it's required.

• I didn't do anything fancy to solo as a metadata attribute, which means it behaves like others (specifically, skip. This means it has the same "inheritance" rules as skip (tests and groups inherit value from containing group). For example:

  $ cat b.dart
  import 'lib/test.dart';
  
  void main() {
    group('group1', () {
      test('test A', () { expect('a', equals('a')); });
      test('test B', () { expect('a', equals('a')); });
    });
  
    solo_group('group2', () {
      test('test C', () { expect('a', equals('a')); });
      solo_test('test D', () { expect('a', equals('a')); });
    });
  }
  $ dart b.dart
  00:00 +0: group1 test A
  00:00 +0 ~1: group1 test B
  00:00 +0 ~2: group2 test C
  00:00 +1 ~2: group2 test D
  00:00 +2 ~2: All tests passed!
  

  test A and test B are skipped; simple enough. test D is run; no surprise there. test C is the only interesting case: it is run, even though it is a non solo test, sitting next to a solo test, because it inherits group2's solo status.

• Other cases are considered in the tests, in this PR.

[nex3]

We've decided not to go in this direction.

[matanlurey]

Can we have some background why? This is still my #1 issue with package:test, and we've had clients outright refuse to convert from unittest to test because they want this exact functionality.