[gdb/testsuite] Add xfail case in gdb.python/py-record-btrace.exp

[gdb/testsuite] Add xfail case in gdb.python/py-record-btrace.exp

I came across:
...
gdb) PASS: gdb.python/py-record-btrace.exp: prepare record: stepi 100
python insn = r.instruction_history^M
warning: Non-contiguous trace at instruction 1 (offset = 0x3e10).^M
(gdb) FAIL: gdb.python/py-record-btrace.exp: prepare record: python insn = r.i\
nstruction_history
...

I'm assuming it's the same root cause as for the already present XFAIL.

Fix this by recognizing above warning in the xfail regexp.

Tested on x86_64-linux, although sofar I was not able to trigger the warning
again.

Approved-By: Markus T. Metzger <markus.t.metzger@intel.com>