[debug] Reserve the trigger in HwbpManual

After riscv-collab/riscv-openocd#1111 is merged,
the registers a user wishes to have direct control of should be
reserved.
This is the case in `HwbpManual`.

The test still works with older OpenOCD versions, since no exception
is generated when a command (`riscv reserve_trigger` in this case)
is not found.

debug/gdbserver.py

@@ -711,6 +711,8 @@ def test(self):
             self.gdb.p("$tdata1=0")
             tselect += 1
 
+        self.gdb.command(f"monitor riscv reserve_trigger {tselect}")
+
         # The breakpoint should be hit exactly 2 times.
         for _ in range(2):
             output = self.gdb.c(ops=2)