[debug tests] increase remotetimeout for all spike-based targets (#553)

Spike simulator is very demanding to CPU resources. This causes debug
tests to sporadically fail on slower machines. Increasing of gdb's
`remotetimeout` should get rid of such failures, unless we run the
testsuite on a potato.

The only downside is that if OpenOCD is broken, tests can run longer.
However, I think this is the sacrifice we can make, since execution time
is not affected if everything works as expected.