gdb/testsuite/dap: make dap_request_and_response not catch / issue test result

gdb/testsuite/dap: make dap_request_and_response not catch / issue test result

Following some of my changes, dap_request_and_response was failing and I
didn't know why.  I think it's better to make it not catch any
exception, and just make it do a simple "send request, read response".
If an exception is thrown while sending a request or reading a response,
things are going really badly, it's not like we'll want to recover from
that and continue the test.

Change-Id: I27568d3547f753c3a74e3e5a730d38a8caef9356