[lit] Use 10 minute timeout by default.

Lit has the ability to set a timeout for individual tests. This patch
enables that functionality with a default of 10 minutes.

Currently we rely on the bots to kill the whole test suite. However this
doesn't tell us which test caused the timeout. Furthermore, when running
the test suite during development, I have to manually kill the tests
that time out to get the lit output at then end. This fixes both
inconveniences.

llvm-svn: 357555