Avoid timeouts with gnat-llvm in gdb.ada/operator_call.exp

While working on gnat-llvm, gdb.ada/operator_call.exp has many
timeouts.  This happens because gnat-llvm's DWARF output is still
incomplete, and so gdb emits an unexpected error in this test.

This patch improves the test by having it recognize this output and
issue a failure rather than a timeout.  This greatly speeds up
testing.