Fix gdb.multi/multi-re-run.exp with native-gdbserver

Fix gdb.multi/multi-re-run.exp with native-gdbserver

The new exec-file-mismatch feature reveals that when running
gdb.multi/multi-re-run.exp against --target_board=native-gdbserver,
we've been starting gdbserver with the wrong program:

 (gdb) spawn /home/pedro/brno/pedro/gdb/binutils-gdb/build/gdb/testsuite/../../gdbserver/gdbserver --once localhost:2347 /home/pedro/brno/pedro/gdb/binutils-gdb/build/gdb/te
 stsuite/outputs/gdb.multi/multi-re-run/multi-re-run-2
 Process /home/pedro/brno/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.multi/multi-re-run/multi-re-run-2 created; pid = 6280
 Listening on port 2347
 target remote localhost:2347
 Remote debugging using localhost:2347
 warning: Mismatch between current exec-file /home/pedro/brno/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.multi/multi-re-run/multi-re-run-1
 and automatically determined exec-file /home/pedro/brno/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.multi/multi-re-run/multi-re-run-2
 exec-file-mismatch handling is currently "ask"
 Load new symbol table from "/home/pedro/brno/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.multi/multi-re-run/multi-re-run-2"? (y or n) Quit
 (gdb) FAIL: gdb.multi/multi-re-run.exp: re_run_inf=1: iter=1: running to all_started in runto

The problem is that gdb_reload uses the last loaded file as binary to
spawn, but we load the program for inferior 2 and then switch to
inferior 1 and run it, so the last loaded file is the program for
inferior 2.

Fix this by tweaking last_loaded_file.

gdb/testsuite/ChangeLog:
2020-05-16  Pedro Alves  <palves@redhat.com>

	* gdb.multi/multi-re-run.exp (test_re_run): Switch
	LAST_LOADED_FILE accordingly.