[gdb/testsuite] Dump dap.log.$n to gdb.log when exceptions found

[gdb/testsuite] Dump dap.log.$n to gdb.log when exceptions found

For a patch I submitted, the Linaro CI reported a failure:
...
FAIL: gdb.dap/attach.exp: exceptions in log file
...

I ran the test-case locally, and observed the same FAIL in the gdb.sum file.

I then wanted to confirm that I reproduced the exact same problem, but
realized that I couldn't because there's no way for me to know what exception
occurred, and where, because that information is logged in the dap.log.$n
file, and the Linaro CI only saves the gdb.sum and gdb.log files.

This issue is even worse if only the CI can reproduce a FAIL.

Fix this in dap_check_log_file by dumping dap.log.$n to gdb.log when
exceptions are found.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>