tests/plugin: allow memory plugin to do both inline and callbacks

tests/plugin: allow memory plugin to do both inline and callbacks

This is going to be useful for acceptance tests that check both types
are being called the same number of times, especially when icount is
enabled.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210213130325.14781-23-alex.bennee@linaro.org>