gdb/testsuite: use with_timeout_factor in 2 more tests

When running tests on some really slow targets, I got timeout in
gdb.base/funcargs.exp and gdb.base/completion.exp. The usual remedy I
use is to set timeout to higher value in site.exp.

However this does not help in case of aforementioned tests because they
set timeout to fixed value, regardless of whether the timeout has been
increased or not, causing them to fail on some of my machines.

This commit fixes the problem by using with_timeout_factor.

Approved-By: Simon Marchi <simon.marchi@efficios.com>