[gdb/testsuite] Fix gdb.cp/annota{2,3}.exp for native-extended-gdbserver

When running test-cases gdb.cp/annota{2,3}.exp with target board
native-extended-gdbserver, we run into a few FAILs, due to the test-cases
trying to match inferior output together with gdb output.

Fix this by ignoring the inferior output in this case.

Tested on x86_64-linux.