Add run_test.sh cmdline option for timeout

Tests implicitly run with a 3-minute timeout. Add an option for this
duration to be overridden. At the same time, fix a long-standing issue
of tests timing out under GDB by changing the default value to 0 if
invoked with -G.

Change-Id: I5ecf12d53bc9a9932c7a580893f0b6df93ca4ff4