[gdb/testsuite] Fix gdb.base/multi-forks.exp timeout with -readnow

[gdb/testsuite] Fix gdb.base/multi-forks.exp timeout with -readnow

When running test-case gdb.base/multi-forks.exp with target board readnow, we
run into:
...
FAIL: gdb.base/multi-forks.exp: run to exit 1 (timeout)
...

Fix this by using exp_continue.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-27  Tom de Vries  <tdevries@suse.de>

	* gdb.base/multi-forks.exp: Use exp_continue to fix timeout.