Update expected info threads error messages in gdb.multi/tids.exp

Update expected info threads error messages in gdb.multi/tids.exp

We currently have these FAILs:
...
FAIL: gdb.multi/tids.exp: two inferiors: info threads -1
FAIL: gdb.multi/tids.exp: two inferiors: info threads -$one
...
because we're expecting:
...
Invalid thread ID: -1
...
but instead we have:
...
Unrecognized option at: -1
...

This error message for info threads has changed since commit 54d6600669
'Make "info threads" use the gdb::option framework'.

Update the test accordingly.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-07-24  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24831
	* gdb.multi/tids.exp: Update error messages for info threads.