[lit] Implement support of per test timeout in lit.

Description

[lit] Implement support of per test timeout in lit.

This should work with ShTest (executed externally or internally) and GTest
test formats.

To set the timeout a new option `--timeout=` has
been added which specifies the maximum run time of an individual test
in seconds. By default this 0 which causes no timeout to be enforced.

The timeout can also be set from a lit configuration file by modifying
the `lit_config.maxIndividualTestTime` property.

To implement a timeout we now require the psutil Python module if a
timeout is requested. This dependency is confined to the newly added
`lit.util.killProcessAndChildren()`. A note has been added into the
TODO document describing how we can remove the dependency on the
`pustil` module in the future. It would be nice to remove this
immediately but that is a lot more work and Daniel Dunbar believes it is
better that we get a working implementation first and then improve it.

To avoid breaking the existing behaviour the psutil module will not be
imported if no timeout is requested.

The included testcases are derived from test cases provided by
Jonathan Roelofs which were in an previous attempt to add a per test
timeout to lit (http://reviews.llvm.org/D6584). Thanks Jonathan!

Reviewers: ddunbar, jroelofs, cmatthews, MatzeB

Subscribers: cmatthews, llvm-commits

Differential Revision: http://reviews.llvm.org/D14706