…python-3.6

…and-line

Snapshot errors
---------------

The constructor of `IntegSnapshotRunner` calls `loadManifest()`, which
on my computer happened to fail and stopped the entire test suite
because this error happened outside the `try/catch` block.

Move it inside the try/catch block, needed a bit of refactoring to
make sure we could still get at the test name.

`--from-file`
-------------

Instead of having `--from-file` require a JSON file with its own
structure, interpret it as a text file which gets treated exactly
the same as the `[TEST [..]]` arguments on the command line.

This still allows for the `--exclude` behavior by setting that flag
on the command-line.

Refactoring
-----------

Moved the logic around determining test names and directories into a
class (`IntegTest`) which is a convenience class on top of a static data
record (`IntegTestInfo`). We pass the data record to worker threads.

…-python-3.6

# Conflicts:
#	packages/@aws-cdk/integ-runner/lib/runner/integration-tests.ts
#	packages/@aws-cdk/integ-runner/lib/workers/extract/extract_worker.ts
#	packages/@aws-cdk/integ-runner/lib/workers/integ-snapshot-worker.ts