gdb/testsuite/dap: make dap_wait_for_event_and_check return preceding messages

gdb/testsuite/dap: make dap_wait_for_event_and_check return preceding messages

In the following patch, I change gdb.dap/basic-dap.exp such that after
waiting for some event, it checks if it received another event
meanwhile.  To help with this, make dap_wait_for_event_and_check and
_dap_dap_wait_for_event return a list with everything received before
the event of interest.  This is similar to what
dap_check_request_and_response returns.

Change-Id: I85c8980203a2dec833937e7552c2196bc137935d