bash_app walltime does not kill process

[Ben Clifford]

Describe the bug
(from reading the source code, not from testing in practice...)

When a bash_app times-out its walltime, nothing kills the underlying process: proc.wait raises a timeout exception but does not kill the underlying process (tree).

discovered while reviewing @khk-globus PR #2822 tidying walltime test

Expected behavior
Walltime expiry should kill the underlying process

Environment
parsl master 9364ea5