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

gdb/testsuite/lib/dap-support.exp

@@ -102,7 +102,7 @@ proc _dap_send_ton {obj} {
     set json [namespace eval ton::2json $obj]
     # FIXME this is wrong for non-ASCII characters.
     set len [string length $json]
-    verbose ">>> $json"
+    verbose -log ">>> $json"
     send_gdb "Content-Length: $len\r\n\r\n$json"
 }