`withinSandbox` is designed for tests that need to act on the
filesystem. It creates a temporary directory and passes it to the
function so that it can do whatever it needs to do within the directory
in a safer fashion.

The name of the sandbox directory is generated from the current time to
ensure that each one is unique. However, this causes problems when Jest
is running more than one test file, each of which make use of the
sandbox. Jest runs test files in parallel, so paired with the naming —
and the fact that time can be frozen in tests — it is possible for two
tests to create and use the same sandbox simultaneously. `withinSandbox`
double-checks that the directory it would have created does not already
exist, so when the first test creates the directory it will cause the
second test to fail. Also, since `withinSandbox` removes the sandbox
directory after it runs its function, this will also cause the second
test to fail if it's still running and using that directory.

To fix this, this commit changes `withinSandbox` to use a UUID to name
the sandbox directory instead of the current time.