gdb/testsuite/dap: write requests to gdb.log

This helps following what happens when reading gdb.log.  The downside is
that it becomes harder to tell what text is from GDB and what text is
going to GDB, but I think that seeing responses without seeing requests
is even more confusing.  At least, the lines are prefix with >>>, so
when you see this, you know that until the end of the line, it's
something that was sent to GDB, and not GDB output.

Change-Id: I1ba1acd8b16f4e64686c5ad268cc41082951c874