[binutils-gdb] btrace: increase buffer size for exception test

[sergiodj+buildbot@redhat.com]

*** TEST RESULTS FOR COMMIT 11cc1b74538286b8bc775057fe1300707b3e3520 ***

Author: Markus Metzger <markus.t.metzger@intel.com>
Branch: master
Commit: 11cc1b74538286b8bc775057fe1300707b3e3520

btrace: increase buffer size for exception test
The trace for throwing and catching an exception can be quite big.
Increase the buffer size to avoid spurious fails.

testsuite/
	* gdb.btrace/exception.exp: Increase BTS buffer size.