[gdb/testsuite] Update expected _gdb_major/_gdb_minor in default.exp

Now that commit "225f296a Change gdb/version.in to 9.0.50.DATE-git (new
version numbering scheme)" has changed the gdb version number, we see:
...
FAIL: gdb.base/default.exp: show convenience ($_gdb_major = 8 not found)
...

Fix this by updating the expected _gdb_major/_gdb_minor to 9.1.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-10-07  Tom de Vries  <tdevries@suse.de>

	* gdb.base/default.exp: Expect _gdb_major/_gdb_minor to be 9.1.