[gdb/testsuite] Fix UNRESOLVED in gdb.server/server-kill-python.exp

[gdb/testsuite] Fix UNRESOLVED in gdb.server/server-kill-python.exp

The test-case gdb.server/server-kill-python.exp runs fine by itself:
...
Running src/gdb/testsuite/gdb.server/server-kill-python.exp ...

                === gdb Summary ===

nr of expected passes            3
...

But if we run f.i. gdb.server/file-transfer.exp before it, we get instead:
...
Running src/gdb/testsuite/gdb.server/server-kill-python.exp ...
ERROR: GDB process no longer exists

                === gdb Summary ===

nr of expected passes            13
nr of unresolved testcases       1
...

We can see the origin of the problem here:
...
spawn gdbserver --once localhost:2347 \
  build/gdb/testsuite/outputs/gdb.server/file-transfer/file-transfer \
  build/gdb/testsuite/outputs/gdb.server/server-kill-python/server-kill-python^M
Process build/gdb/testsuite/outputs/gdb.server/file-transfer/file-transfer
  \ created; pid = 9464^M
Listening on port 2347^M
...

The spawn of the gdbserver for the server-kill-python test-case gets as
executable argument the file-transfer binary.

This is caused by proc gdbserver_spawn attempting to load the exec file in
$file_last_loaded.  This is something that is meant to load the same exec in
the gdbserver that was earlier loaded into gdb.

In this test-case however, nothing has been loaded into gdb by the test-case,
and consequently we load the file that was loaded into gdb in the previous
test-case.

Fix this by unsetting $file_last_loaded in gdb_init.

Build and reg-tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-02-11  Tom de Vries  <tdevries@suse.de>

	PR testsuite/25488
	* lib/gdb.exp (gdb_init): Unset $file_last_loaded.

Change-Id: Ic385e08cbd34cbf85518720cf5695b4ff6619f4b