Originally the integ-runner was built to work with the "legacy"
integration tests since that is all we currently have. Since then
we have published the `integ-tests` library which adds support
for the integ manifest. This PR adds additional coverage for the
integ manifest.

To make the implementation a little cleaner I've extracted the logic
that deals with the manifest into `IntegTestCases` and
`LegacyIntegTestCases` classes. This allows the runner to just deal with
the manifest and not have to worry about handling where it came from.

I've also split up the `runners.ts`/`runners.test.ts` into
`integ-test-runner.ts`, `runner-base.ts`, and `snapshot-test-runner.ts`

Coverage added:

1. `stackUpdateWorkflow` - This was moved from the individual `testCase`
   to the manifest. This means that we will enable or disable the update
   workflow at the test level instead of the individual test case level.
   This also required a bump in the schema version.
2. `diffAssets`
3. `hooks`