Refactor local_test.go to make tests (more) idempotent

[rfay]

What happened (or feature request):

As noted in #337 (comment) there are some serious problems with the structure of local_test.go:

• TestMain sets up the workspaces and then removes them after tests.
• However, TestMain does not shut down the sites or even try to do so. It could at least use the same techniques as TestLocalRemove and TestCleanupWithoutCompose to clean up the containers.
• The result is 3 nonworking sites (which do not show up on ddev list, apparently because the workspace is gone)
• TestLocalExec is dependent on a database having been loaded by TestLocalImportDb

What you expected to happen:

• Each test should be able to be run independently
• Each test run should leave the system in a predictable state
• ddev list should report sites even when their workspace is trashed, this will likely happen in real life.

Version: Please include the output of ddev version and the project's .ddev/config.yaml.

How to reproduce this:

make testpkg TESTARGS="-run TestLocalStart"

Anything else do we need to know:

Related source links or issues: