tests/tcg: better trap gdb failures

tests/tcg: better trap gdb failures

It seems older and non-multiarach aware GDBs might not fail gracefully
when faced with something they don't know. For example when faced with
a target XML for s390x the Ubuntu 18.04 gdb will generate an internal
fault and prompt for a core dump.

Work around this by invoking GDB in a more batch orientated way and
then trying to filter out between test failures and gdb failures.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200430190122.4592-4-alex.bennee@linaro.org>