Hanging tests due to exceptions in artifact cache

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/811)
In GitLab by [[Gitlab user @jmacarthur]](https://gitlab.com/jmacarthur) on Dec 7, 2018, 10:18

## Summary

Likely only to affect developers but very annoying. Exceptions in the Artifact cache cause the BuildStream tests to lock up. They cannot be interrupted with `^C` and need a `kill -9` in another terminal to kill them.

## Steps to reproduce

Simulate a missing file in `CASCache._get_subdir`, like this:

```
diff --git a/buildstream/_artifactcache/cascache.py b/buildstream/_artifactcache/cascache.py
index 9ca757d4..29cb84a0 100644
--- a/buildstream/_artifactcache/cascache.py
+++ b/buildstream/_artifactcache/cascache.py
[[Gitlab user @]](https://gitlab.com/)[[Gitlab user @]](https://gitlab.com/) -797,6 +797,7 [[Gitlab user @]](https://gitlab.com/)[[Gitlab user @]](https://gitlab.com/) class CASCache():

def _get_subdir(self, tree, subdir):
head, name = os.path.split(subdir)
+ raise CASError("Subdirectory {} not found".format(name))
if head:
tree = self._get_subdir(tree, head)
```

(This exception is raised at the end of this function, and can genuinely be raised if an badly-constructed artifact was placed in the cache.)

Now run the test:

`/setup.py test --addopts "tests/artifactcache/pull.py::test_pull --integration -s"`

This will halt at 'pull'.

## What is the current bug behavior?

Tests lock up and can only be cleared with SIGKILL.

## What is the expected correct behavior?

Details of the exception being raised are visible to the tester.

## Possible fixes

While we can't simply remove it, `ExitStack` in `tests/testutils/runcli.py` is likely to be relevant. In my case, I tracked down the underlying exception by removing ExitStack from `Cli.run` temporarily. This allows the exception text and backtrace to appear on the test output.

* BuildStream version affected: /milestone %BuildStream_v1.3

----

Start with tests/testutils/runcli.py and its ExitStack handling, then inspect buildstream/_artifactcache/cascache.py and reproduce the problem with /setup.py test --addopts "tests/artifactcache/pull.py::test_pull --integration -s". Done means the test no longer hangs when an artifact-cache exception occurs and the exception details and backtrace are visible in the test output.

Written by the indexing model from the issue text.