Support multi-phase test runs.

[bsutton]

The conduit project has a number of tests that cannot be run in parallel.

One suggested approach is that we allow a multi-stage testing process.

Each stage would run using a specific set of tags.

This will probably need to be done via a config file as the cli arguments would be a bit complex

pub_release -staged=single,parallel

Something like the above might work. This would create two stages on run with the 'single' tag and one run with the 'parallel' tag.

We would need to allow quoting of these as dart tests tags support tag expressions.
e.g.
single && !windows

We could create a config file pub_release.yaml with the following sections:

stages:
stage:
tag: single
thread: 1
stage: parallel
threads: 4
stage: single && !windows

One issue with the above approach is that we may need different switches for each stage.

For example conduit needs different -j1 switches pass in each stage.