Use the .py file in gdb.dap/pause.exp

Tom de Vries pointed out that the gdb.dap/pause.exp test writes a
Python file but then does not use it.  This patch corrects the
oversight.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31354
Reviewed-By: Tom de Vries <tdevries@suse.de>

gdb/testsuite/gdb.dap/pause.exp

@@ -134,7 +134,7 @@ dap_read_response cancel $cancel_id
 # does not continue the inferior) can be canceled.
 #
 
-write_file py "while True:\n  pass"
+set gdbfile [write_file py "while True:\n  pass"]
 set cont_id [dap_send_request evaluate \
 		 [format {o expression [s "source %s"] context [s repl]} \
 		      $gdbfile]]