Increase the timeout for the inttest/proto_gpb

On my slow old netbook with a 1.6GHz CPU, this inttest
takes about 40 seconds to run, of which network activity
is about 3s, the default timeout of retest is 30s.
Add a generous margin to avoid unnecessary timeouts.
Merge pull request #399 from tomas-abrahamsson/gpb-inttest-timeout

Increase the timeout for the inttest/proto_gpb
compiler and xref respect 'keep_going' flag
add tests for 'compiler keep_going'